Printable page generated Tuesday, 21 May 2024, 5:31 PM Use 'Print preview' to check the number of pages and printer settings. Print functionality varies between browsers.
This module will introduce how antibiotics work and how bacteria acquire and transmit resistance.
You will start by exploring how antibiotics can exert powerful antibacterial effects but be generally well tolerated by people and animals, focusing on the different modes of antibiotic action. You will then consider how bacteria develop resistance in order to protect themselves from antibiotics. You will explore the main mechanisms of antibiotic resistance as well as the differences between intrinsic and acquired resistance, including how acquired resistance is transferred.
You may be familiar with the terms ‘antimicrobial’, ‘antibacterial’ and ‘antibiotic’. Each has a slightly different meaning, but in this module, we will use the term ‘antibiotic’ to refer to any drug that is active against bacteria.
After completing this module, you will be able to:
explain how antibiotics work against bacterial pathogens
state what is meant by the term ‘antibiotic resistance’
explain that antibiotic resistance is a natural phenomenon that protects bacteria from hostile environments
explain how resistance mechanisms arise, and how resistance can be passed on during bacterial replication
describe the horizontal gene transfer mechanisms that allow antibiotic resistance to be transferred between bacteria
apply scientific terminology when explaining how antibiotic resistance relates to your current work.
Activity 1: Assessing your knowledge and skills
Timing: Allow about 10 minutes
Before you begin this module you should take a moment to think about the learning outcomes and how confident you feel about your knowledge and skills in these areas. Do not worry if you do not feel very confident in some skills – they may be areas that you are hoping to develop by studying these modules.
Now use the interactive tool to rate your confidence in these areas using the following scale:
5 Very confident
4 Confident
3 Neither confident nor not confident
2 Not very confident
1 Not at all confident
Try to use the full range of ratings shown above to rate yourself:
Active content not displayed. This content requires JavaScript to be enabled.
Although this is an introductory course on antibiotic resistance, it assumes that you have a basic understanding of DNA and proteins. If you are unfamiliar with these concepts, you might want to try our free OpenLearn course What do genes do? and listen to our set of audios on DNA, RNA and protein formation before you begin this course.
Activity 2: New terms from Introducing antimicrobial resistance
Timing: Allow about 10 minutes
As you work through the module, put together a personal list of terms that are new to you. Make sure that you feel confident with what they mean and that you can apply them to your own working practice.
Have you heard any of these terms mentioned in your day-to-day work?
What was the context that they were used in?
Can you think of how you would use some of these terms in your day-to-day work?
Drugs should demonstrate ‘selective toxicity’; that is, they target the disease-causing organism while causing no or minimal harm to the patient. This principle, proposed by Paul Erlich in 1900, is still firmly entrenched in medical and veterinary research and practice today (Valent et al., 2016).
Underpinning selective toxicity are the differences between bacterial pathogens and human or animal cells. It is these differences that antibiotics (and other drugs) exploit to exert their specific effects.
There are two distinct types of cell. Bacteria are prokaryotes while human and animal cells are eukaryotes. Figure 1 below shows the basic structures of each type of cell.
Figure 1 Schematic diagram of a typical (a) eukaryotic animal cell and (b) prokaryotic bacterial cell (not drawn to scale).
Show description|Hide description
Part (a) is a simplified 3D diagram of a eukaryotic animal cell. The cell is bounded by a thin, flexible cell membrane. Inside is the nucleus, which is also membrane-bound and contains the DNA. Also present are membrane-bound components called organelles, and many tiny structures called ribosomes. The fluid interior of the cell is called the cytosol. The ribosomes are either free in the cytosol or attached to the outside of membranous sacs. Part (b) is a simplified 3D diagram of a prokaryotic bacterial cell. A thick cell wall surrounds the cell membrane. There is no nucleus nor are there any interior membranes; the ‘naked’ DNA and all the ribosomes are free in the cytosol.
Figure 1 Schematic diagram of a typical (a) eukaryotic animal cell and (b) prokaryotic bacterial cell (not drawn to scale).
Although there are similarities between the two cell types, eukaryotic cells are structurally much more complex. Prokaryotes and eukaryotes carry out the same essential processes necessary for survival, such as making new proteins, metabolism and reproduction, but these processes are not identical. For example, different proteins might be produced or different enzymes used to drive key chemical reactions.
Antibiotics are selectively toxic because they target structural features or cellular processes in the bacterial pathogen that are different or lacking in the host’s cells.
1.2 Potential bacterial targets for antibiotics
In Activity 3 you will discover which essential cell processes in the bacterial pathogen are potential targets for antibiotics.
Activity 3: When are bacteria vulnerable to antibiotics?
Timing: Allow about 20 minutes
Watch the video about key bacterial cell processes and answer the related questions. You can pause the video to work through this activity at your own pace.
If you are unable to watch this video, or any other video in this course, you can view a transcript of the content by clicking on ‘Show transcript’ below.
How do antibiotics work? Pathogenic bacteria in the body cause infections, which can be treated by antibiotics.
TEXT ON SCREEN: Antibiotics can be bacteriostatic or bactericidal. Statis = to stop, Cidal = to kill
Antibiotics can be bacteriostatic or bactericidal. Bacteriostatic antibiotics slow the growth of bacteria by interfering with the processes the bacteria need to multiply. These processes include:
DNA replication.
TEXT ON SCREEN: DNA replication. Typically a bacterial DNA takes the form of a single, circular DNA molecule called a chromosome. Along the length of the chromosome are many short sections of DNA called genes which carry the instructions to make one of the thousands of proteins that cells need to grow and function. Before the bacterial cell divides, the DNA must make a copy of itself (replicate) so that each daughter cell can receive a copy of this chromosome.
Q1: Suggest a likely consequence for the cell if DNA replication is blocked.
Answer: Blocking DNA replication would impair cell division and kill the bacterial cell.
Metabolism, e.g., enzyme activity.
TEXT ON SCREEN: Metabolism, e.g. enzyme activity. Metabolism refers to the chemical reactions that occur within a cell to ensure an organism obtains the energy and nutrients it needs. Enzymes are proteins that are required for metabolism; they bind in a specific manner to another molecule (the substrate) in order to help it undergo a chemical reaction.
Protein production.
TEXT ON SCREEN: Protein production. Proteins, which are composed of amino acid building blocks, are synthesised in two stages. First, the instructions carried by the gene are transferred to a messenger ribose nucleic acid molecule (mRNA) and taken to a ribosome for processing. Second, the instructions are used to create a long chain of amino acids – the order of the amino acid building blocks is unique to the protein being made. Once complete, the amino acid chain folds up into a complex, three-dimensional protein.
Q2: Which stage or stages of protein synthesis could be targeted by antibiotics?
Answer: Interference with either stages of protein synthesis could result in faulty enzymes and/or structural proteins.
Q3::DNA replication, metabolic reactions and protein synthesis also occur in eukaryotic cells. Suggest why antibiotics that target these bacterial processes demonstrate selecting toxicity.
Answer: Although cellular processes of prokaryotic and eukaryotic cells have many similarities, antibiotics are selected for clinical use that target those process that are wholly or partly unique to the bacterial pathogen. This minimises the risk of side-effects in the patient.
Bactericidal antibiotics kill the bacteria. For example, by preventing the bacteria from making a cell wall.
TEXT ON SCREEN: Q4: What might happen to a cell that can no longer make a cell wall?
Answer: Bacterial cells that lack a cell wall are in danger of osmotic damange and lysis.
Penicillins are bactericidal. Penicillins include Penicillin V for sore throats, amoxicillin for chest infections, and fluctoxacillin for skin infections. Antibiotics can be so-called broad-spectrum, affecting many different bacteria in your body, including useful bacteria in your gut.
Some antibiotics are more narrow-spectrum, only affecting one or two types of bacteria. It is better to use narrow-spectrum antibiotics where possible. Most antibiotics have no effect on your immune system.
Antibiotics do not work on viruses, because viruses have a different structure to bacteria. Viruses incorporate themselves into a host cell in your body in order to multiply. Bacteriostatic antibiotics that affect bacterial DNA, metabolism, or protein production do not attack body cells. And therefore, do not slow the growth of viruses.
Viruses do not have a cell wall. And therefore, bactericidal antibiotics that act on cell walls cannot kill viruses.
(a) Suggest a likely consequence for the cell if DNA replication is blocked.
Answer
Blocking DNA replication would impair cell division and kill the bacterial cell.
(b) Which stage or stages of protein synthesis could be targeted by antibiotics?
Answer
Interference with either stage of protein synthesis could result in faulty enzymes and/or structural proteins.
(c) DNA replication, metabolic reactions and protein synthesis also occur in eukaryotic cells. Suggest why antibiotics that target these bacterial processes demonstrate selective toxicity.
Answer
Although cellular processes of prokaryotic and eukaryotic cells have many similarities, antibiotics are selected for clinical use that target those processes that are wholly or partly unique to the bacterial pathogen. This minimises the risk of side effects in the patient.
(d) What might happen to a cell that can no longer make a cell wall?
Answer
Bacterial cells that lack a cell wall are in danger of bursting if too much water moves into the cell by osmosis
(e) Why do antibiotics that target cell wall synthesis leave human and animal cells unharmed?
Answer
Human and animal cells lack a cell wall.
(f) A relatively small number of antibiotics target the bacterial cell membrane. Such antibiotics are often highly toxic to the host. Can you suggest a reason for this?
Answer
The membrane of animal and human cells has a very similar structure to that of bacteria. The potential for such antibiotics to adversely affect eukaryotic cells is therefore greater and these antibiotics generally demonstrate poor selective toxicity. This increases the risk of harmful side effects for the patient.
1.3 Types of antibiotic
Antibiotics may be active against a wide range of bacteria (broad-spectrum) or just a few types (narrow-spectrum). Additionally, antibiotics either kill bacterial cells (bactericidal) or stop them growing and dividing (bacteriostatic).
Factors that determine the spectrum of antibiotic activity include:
ability to penetrate the bacterial cell – since most bacterial targets are located in the cell’s interior
how widespread the target is among different bacterial species
bacterial resistance to the antibiotic.
1.3.1 Gram-positive and Gram-negative bacteria
Can you remember the importance of the cell wall to bacterial cells?
It is a protective outer layer that provides mechanical support to the cell. It also prevents harmful surges of water moving into the cell by osmosis, which could cause it to burst (lysis).
Bacteria are divided into two main groups based on how the cell wall appears when they are stained using Gram straining. This procedure allows the composition of the wall to be visualised. Some ‘atypical’ bacteria, such as Mycoplasma and Chlamydia, cannot be seen by Gram staining and so cannot be categorised in this way, but these will not be considered in this module.
In Gram-positive bacteria, the cell wall has a thick peptidoglycan layer which is relatively porous, allowing substances to pass through it quite easily. One of the components of the Gram stain, crystal violet, is absorbed into the peptidoglycan layer, staining it a dark purple colour.
In Gram-negative bacteria, this peptidoglycan layer is greatly reduced and is further protected by a second, outer membrane (Figure 2). The outer membrane does not allow crystal violet to reach the peptidoglycan layer and so these cells appear pink under the microscope due to the counterstain – usually safranin or neutral red.
(For more information on the Gram stain and other techniques used in the laboratory to help identify bacteria you could visit the module entitled Isolating and identifying bacteria.)
Figure 2 Arrangement of the cell wall in (a) Gram-positive and (b) Gram-negative bacteria.
Show description|Hide description
This diagram shows the differences in cell wall structure between Gram-positive and Gram-negative bacteria. In the Gram-positive bacteria in (a) the peptidoglycan is a thick external layer shown in brown, while in the Gram-negative bacteria in (b) the peptidoglycan layer is much thinner and is surrounded by an outer membrane of lipopolysaccharide and protein (as a green wavy line). The inner membrane in (a) and (b) (shown as a double green line) is separated from the peptidoglycan layer by the periplasmic space.
Figure 2 Arrangement of the cell wall in (a) Gram-positive and (b) Gram-negative bacteria.
This second, outer membrane of Gram-negative bacteria is an effective barrier, regulating the passage of large molecules such as antibiotics into the cell. In contrast, the thick, porous peptidoglycan layer in the cell wall of Gram-positive bacteria gives greater access to antibiotics, allowing them to more easily penetrate the cell and/or interact with the peptidoglycan itself.
1.3.2 Activity against Gram-positive and Gram-negative bacteria
Narrow-spectrum antibiotics are effective against either Gram-positive or Gram-negative bacteria, whereas broad-spectrum antibiotics are effective against both types.
Not all Gram-positive and/or Gram-negative bacteria are affected by a single antibiotic. Can you suggest a possible reason for this?
This is because of the specificity of the antibiotic/bacterial target interaction, whether the bacterial species has the target in question and whether the bacteria are resistant to the antibiotic.
1.3.3 Bactericidal versus bacteriostatic antibiotics
While some antibiotic classes have consistent antibacterial effects, such as β-lactams which are nearly always bactericidal, the activity of other classes may depend on the dose of antibiotic prescribed or how long the treatment lasts. For example, fluoroquinolones and aminoglycosides, while usually bactericidal, may be bacteriostatic when used at low concentration.
You should by now have a good idea of how antibiotics interact with bacterial cells. You will now consider what happens to the bacterial population as a whole when antibiotics are administered.
In nature, bacterial growth follows a typical pattern shown in Figure 3. The growth curve comprises four phases:
The lag phase: Bacteria are adapting to their environment; nutrients are plentiful and the cells grow in size. Cell number remains relatively constant, balanced by the deaths of some cells and division of others.
The exponential/logarithmic (log) phase: This phase marks a big increase in cell numbers. Maximum growth rate is achieved, with a constant doubling of the bacterial population. Growth then slows as nutrients become depleted and bacterial waste products build up to toxic levels.
The stationary phase: The bacteria enter this phase when the number of new cells equals the number of cells dying. The total number of cells in the population remains constant.
The death/decline phase: Unless nutrients are replenished and waste products are removed, the bacteria progress to the death phase. More cells die than are produced and the number of cells in the population declines; however, some cells may remain viable (capable of surviving).
Figure 3 Graph of bacterial growth showing how the number of cells changes with time in a culture in which the bacteria are reproducing by binary fission.
Show description|Hide description
This figure is a simple graph in which the horizontal axis is labelled time and the vertical axis is labelled number of cells. The four phases of the growth curve are distinguished using background shading of different colours. In the lag phase (yellow), cell number increases very slowly with time; cell number increases progressively rapidly during the exponential phase (red), levels off to a constant value in the stationary phase (blue), then falls during the death phase (grey).
Figure 3 Graph of bacterial growth showing how the number of cells changes with time in a culture in which the bacteria are reproducing ...
Bacteria are at their most susceptible to antibiotic attack when they are actively growing. You will now consider what happens to a bacterial culture when antibiotics are introduced during this exponential phase of growth.
Activity 4: Effect of antibiotics on bacterial growth
Timing: Allow about 10 minutes
Bacteriostatic antibiotics
(a) A typical growth curve is shown in Figure 4a.
Figure 4a A typical growth curve.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the presence of antibiotic A. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards. The first arrow indicates the timepoint at which antibiotic A was added. After the first arrow the line flattens out and remains constant.
Figure 4a A typical growth curve.
Figure 4b shows the normal growth curve of a bacterium which is sensitive to the bacteriostatic antibiotic ‘A’. What will happen to the bacterial growth rate when ‘A’ is added to the culture in high concentration where all other growth conditions are optimal?
Figure 4b Normal growth curve of bacterium in the absence of antibiotic A.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the absence of antibiotic A. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards up until a point at which antibiotic A was added, which is indicated by an arrow. At this point the graph stops.
Figure 4b Normal growth curve of bacterium in the absence of antibiotic A.
Answer
Figure 4c The bacterial population remains constant as the cells are prevented from growing and dividing.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the presence of antibiotic A. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards. The first arrow indicates the timepoint at which antibiotic A was added. After the first arrow the line flattens out and remains constant.
Figure 4c The bacterial population remains constant as the cells are prevented from growing and dividing.
(b) Now predict what will happen to bacterial growth if antibiotic A is removed from the culture at the point indicated on the graph in Figure 4d.
Figure 4d The bacterial population remains constant as the cells are prevented from growing and dividing.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the presence of antibiotic A. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards. The first arrow indicates the timepoint at which antibiotic A was added. After the first arrow the line flattens out and remains constant. The second arrow indicates when antibiotic A was removed. At this point the graph stops.
Figure 4d The bacterial population remains constant as the cells are prevented from growing and dividing.
Answer
Figure 4e As the bacteria are still alive and nutrients are plentiful, the cells can now grow and divide, and the population starts to increase again.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the presence of antibiotic A. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards. The first arrow indicates the timepoint at which antibiotic A was added. After the first arrow the line flattens out and remains constant. The second arrow indicates when antibiotic A was removed. After this point the line continues to slope upwards.
Figure 4e As the bacteria are still alive and nutrients are plentiful, the cells can now grow and divide, and the population starts to ...
Bactericidal antibiotics
Figure 5a shows the normal growth curve of a bacterium that is sensitive to the bactericidal antibiotic ‘B’. What will happen to the rate of bacterial growth when B is added to the culture in high concentration? Again, you should assume that growth conditions are otherwise optimal.
Figure 5a Normal growth curve of the bacterium in the absence of antibiotic B.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the absence of antibiotic B. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards. The arrow indicates the timepoint at which antibiotic B was added. At this point the graph stops.
Figure 5a Normal growth curve of the bacterium in the absence of antibiotic B.
Answer
Figure 5b The number of bacterial cells falls rapidly as the cells are killed.
Show description|Hide description
This figure shows a representation of the growth of a bacterium in the presence of antibiotic B. The horizontal axis is labelled time and the vertical axis is labelled log (no. of viable cells). The line crosses the vertical axis near the bottom before sloping upwards. The arrow indicates the timepoint at which antibiotic B was added. After the arrow the line slopes sharply towards the horizontal axis.
Figure 5b The number of bacterial cells falls rapidly as the cells are killed.
Bactericidal antibiotics kill susceptible bacteria during the exponential phase of growth and help to eliminate the infection.
Bacteriostatic antibiotics stop bacterial growth even though the cells remain viable. This allows time for the host’s immune system to be activated and target the bacterial pathogen – again effecting a cure.
1.4 Antibiotic modes of action
This section focuses on the four main modes of antibiotic action (Figure 6) that lead to inhibition of one of the following:
cell wall synthesis
protein synthesis
nucleic acid synthesis
metabolic reactions
cell membrane function.
Don’t worry if you don’t understand all of these terms, as they will be explained in later sections.
Figure 6 Some of the main antibiotic modes of action.
Show description|Hide description
This figure shows a simplified 3D diagram of a prokaryotic bacterial cells as described in Figure 1 part (b). Each cellular component is labelled with the mode of antibiotic action that affects that structure. The labels contain a table describing the target of this mode of action and the antibiotic that exerts this effect. The first label is ‘Protein synthesis’. One target is small ribosome subunit and the attacking antibiotic is Aminoglycosides. The next target is large ribosome subunit, the attacking antibiotic for which is Oxazolidinones. The second label is ‘Metabolic reactions’. The target is Folic acid synthesis and the attacking antibiotic is Trimethoprim. The next label is ‘Cell wall synthesis’. The target is Peptidoglycan cross-linking and the attaching antibody is β-lactams: penicillins cephalosporins. The final label is ‘Nucleic acid synthesis’. The first target is enzymes which unwind DMA and its attacking antibiotic is Fluoroquinolones. The next target is RNA polymerase, the attacking antibiotic for which is Rifamycins.
Figure 6 Some of the main antibiotic modes of action.
Members of the same class of antibiotics share a characteristic structural feature that determines the drug’s affinity and specificity for target molecules in susceptible bacteria. You will now look in more detail at antibiotics that exemplify each of these modes of action.
1.4.1 Inhibitors of cell wall synthesis
As you saw in Activity 3, the cell wall is essential for normal functioning of the bacterial cell. Antibiotic inhibitors of cell wall synthesis block the production of peptidoglycan, the main component of the cell wall. Cross-linking between peptidoglycan chains forms a strong, mesh-like structure that gives the cell wall structure and rigidity (Figure 7) and protects the underlying cell membrane from osmotic damage when water moving into the cell by osmosis could cause it to burst, or lyse. Osmosis is where water moves through the membrane to try and balance the concentration of water molecules both inside and outside the cell.
Figure 7 Structure and arrangement of peptidoglycan chains in the bacterial cell wall. Peptidoglycan molecules consist of a backbone of carbohydrate units with sets of amino-acid residues attached (yellow). They are cross-linked by bridges (red), providing structure and strength.
Show description|Hide description
The diagram shows two adjacent chains of alternating N-acetylglucosamine and N-acetylmuramic acid units, represented here as large blue balls. Extending downwards from each N-acetylmuramic acid unit is a chain of four amino acid residues, shown here as small yellow balls. The ends of the amino acid chains on adjacent carbohydrate units are joined by glycine cross-bridges, shown here as red bars.
Figure 7 Structure and arrangement of peptidoglycan chains in the bacterial cell wall. Peptidoglycan molecules consist of a backbone of ...
Disruption of the peptidoglycan layer of the cell wall can therefore result in cell lysis (Figure 8).
Figure 8 Lysis of a bacterium with a defective cell wall. (a) Diagram showing the sequence of events that lead to lysis. (b) Light micrograph of S. aureus: a lysed cell on the left and an intact dividing cell on the right.
Show description|Hide description
Part (a) is a schematic diagram showing the sequence of events that lead to the osmotic lysis of a bacterium. Initially the cell wall and the cell membrane beneath it are intact. As water enters by osmosis, the cell wall becomes defective. Eventually the cell contents and surrounding membrane expand through the defective cell wall, the membrane then ruptures and the cell contents spill out; that is, the cell lyses. In the light micrograph in part (b), the intact near-spherical cell appears orange–yellow on the black background; while the lysed cell has collapsed and lost most of its contents and so has a shrivelled shape and appears mostly black.
Figure 8 Lysis of a bacterium with a defective cell wall. (a) Diagram showing the sequence of events that lead to lysis. (b) Light ...
Examples of cell wall synthesis inhibitors are the β-lactam antibiotics. These include penicillin and its derivatives, and the cephalosporins. All β-lactam antibiotics contain a core chemical structure called a β-lactam ring (Figure 9) that determines the mode of action of this class of antibiotics.
Figure 9 Core ring structures of two types of β-lactam antibiotics. The β-lactam ring is shaded pink in each case.
Show description|Hide description
This figure shows the core ring structures of penicillins (top) and cephalosporins (bottom). They both contain a ring that is joined to the common beta-lactam ring (shaded in pink), but the structure of this ring differs between the two classes: the penicillins have a sulphur (S)-containing 5-membered ring with no double bonds; the cephalosporins have a sulphur (S)-containing 6-membered ring with one double bond.
Figure 9 Core ring structures of two types of β-lactam antibiotics. The β-lactam ring is shaded pink in each case.
The β-lactam antibiotics interfere with the formation of the peptidoglycan cross-links by inhibiting the enzymes responsible for cross-linking adjacent molecules in the peptidoglycan layer. The β-lactam antibiotics bind to these enzymes, collectively known as penicillin-binding proteins (PBPs) and prevent them from forming cross-links.
Figure 10 shows what happens when a β-lactam antibiotic, in this case penicillin, binds to an active PBP.
Figure 10 Reaction of penicillin with a PBP. The –NH2 side chain of the PBP reacts with the β-lactam ring of penicillin to form a new side chain. This reaction releases the strain in the β-lactam ring, which remains open.
Show description|Hide description
This is the equation for the reaction of penicillin with a side chain amine group, –NH2, on a protein molecule. The bond between the nitrogen and the carbonyl carbon atom of the β-lactam ring is broken, and this carbon forms an amide, –CONH–, linkage with the amine group on the protein. At the same time, one of the protein’s amine hydrogen atoms is transferred to the N atom that was in the β-lactam ring. The reaction results in deactivation of the protein.
Figure 10 Reaction of penicillin with a PBP. The –NH2 side chain of the PBP reacts with the β-lactam ring of penicillin to form a new ...
The reaction shown in Figure 10 results in a new PBP side chain which is much larger than the original –NH2 group and effectively deactivates the PBP.
By interfering with the formation of peptidoglycan cross-links β-lactam antibiotics weaken the cell wall. In addition, repairs cannot be made to the cell walls, allowing the osmotic pressure inside the cells to increase, eventually leading to lysis.
Later in the module you will learn more about how disrupting the interaction between β-lactam antibiotics and PBP contributes to antibiotic resistance mechanisms.
The glycopeptides (e.g. vancomycin) are another group of antibiotics that inhibit cell wall synthesis, but by a different mechanism.
1.4.2 Inhibitors of protein synthesis
Cells synthesise new proteins in ribosomes that are made up of one large and one small subunit. These subunits differ structurally and chemically between prokaryotic and eukaryotic ribosomes (Figure 11). These differences mean that antibiotics can target bacterial ribosomes but have minimal effect on the ribosomes of the host cell.
Figure 11 Ribosome structure in (a) prokaryotes and (b) eukaryotes. The Svedberg unit (S) indicates the size, shape and density of each subunit.
Show description|Hide description
This figure shows the ribosome structure in prokaryotes (a) and eukaryotes (b). Part (a) shows the prokaryote 70S ribosome which is comprised of a large, 50S, and small, 30S, subunit represented as two blue ovals labelled 50S (top) and 30S (bottom). Part (b) shows the eukaryote 80S ribosome which is comprised of a large, 60S, and small, 40S, subunit represented as two blue ovals labelled 60S (top) and 40S (bottom). The large subunit of both ribosomes is responsible for creating the links in the growing amino acid chain. The small subunit in both ribosomes is where mRNA carrying the instructions for making protein binds. The Svedberg unit (S) indicates the size, shape and density of each subunit.
Figure 11 Ribosome structure in (a) prokaryotes and (b) eukaryotes. The Svedberg unit (S) indicates the size, shape and density of each ...
Several antibiotics that you may be familiar with, such as gentamicin, tetracycline and erythromycin, work in this way. Interfering with ribosome functions means that cells cannot produce essential proteins and will either die or be unable to replicate. Other antibiotics, such as fusidic acid, also inhibit the production of proteins by blocking different parts of the protein synthesis pathway.
1.4.3 Inhibitors of nucleic acid synthesis
Differences between enzymes that carry out the synthesis of nucleic acids in eukaryotic and prokaryotic cells allow antibiotics to target these processes in bacteria.
For example, fluoroquinolones such as ciprofloxacin and levofloxacin (Figure 12) specifically inhibit bacterial enzymes that unwind the DNA double helix, separating the two strands so that the DNA can be copied. If the strands of DNA do not unwind and separate, bacterial replication is blocked.
Figure 12 The fluoroquinolone called ciprofloxacin. Fluoroquinolones all contain the chemical structure highlighted in red.
Show description|Hide description
This figure shows the chemical structure of the fluorquinolone ciprofloxacin. The structure highlighted in red is common to all fluoroquinolones. N = nitrogen, O = oxygen, H = hydrogen.
Figure 12 The fluoroquinolone called ciprofloxacin. Fluoroquinolones all contain the chemical structure highlighted in red.
Another class of antibiotics – rifamycins – inhibit RNA synthesis by binding to and inhibiting an enzyme called RNA polymerase. This enzyme transfers the instructions carried by genes to the intermediary molecule, mRNA. Interference in this process ultimately stops new proteins being made. Rifampicin is a commonly used rifamycin.
1.4.4 Inhibitors of metabolic reactions
Antibiotics that disrupt essential bacterial metabolic pathways are acting as antimetabolites. These chemicals are structurally similar to natural metabolites but just different enough to interfere with normal cell function.
For example, trimethoprim inhibits the synthesis of folic acid, a vitamin that bacteria, unlike humans, must make themselves. Trimethoprim is a structural analogue of dihydrofolic acid, an intermediate compound in the folic acid pathway. Trimethoprim out-competes dihydrofolic acid to react with a specific bacterial enzyme in the pathway, thereby interrupting folic acid synthesis and inhibiting bacterial growth (Figure 13a).
Figure 13a The folic acid pathway. Trimethoprim prevents the enzyme dihydrofolate reductase reacting with the intermediate compound dihydrofolic acid, thereby blocking the pathway at the point shown.
Show description|Hide description
This figure shows the folic acid pathway. Para-aminobenzoic acid and pteridine are converted into dihydrpteroic acid by dihydorpteroate synthetase. Dihydropteroic acid is then converted to dihydrofolic acid by dihydrofolate synthetase. In the final step of the pathway dihydrofolic acid is converted to tetrahydrofolic acid, the active form of folic acid, by dihydrofolate reductase. Trimethoprim prevents the enzyme dihydrofolate reductase reacting with the intermediate compound dihydrofolic acid, thereby blocking the pathway in the final step.
Figure 13a The folic acid pathway. Trimethoprim prevents the enzyme dihydrofolate reductase reacting with the intermediate compound ...
This figure shows the underlying competitive mechanism by which trimethoprim inhibits dihydrofolate reductase. Part (a) shows the mechanism in the absence of trimethoprim represented as an equation in which dihydrofolic acid (in green) and dihydrofolate reductase (in yellow labelled enzyme) interact so that dihydrofolic acid binds to the active site of the dihydrofolate reductase. Part (b) shows the mechanism in the presence of trimethoprim represented as an equation. Trimethoprim (in red) competes with dihydrofolic acid (green) to bind to the enzyme (represented in yellow). When trimethoprim binds to the enzyme dihydrofolic acid is unable to bind and the pathway is interrupted.
Figure 13b The underlying competitive mechanism.
The action of trimethoprim illustrated in Figure 13b exemplifies the specific interaction between antibiotic and bacterial target at a molecular level that disrupts a particular cellular process.
Identify the structure labelled ‘enzyme’.
The enzyme is dihydrofolate reductase.
1.4.5 Inhibitors of cell membrane function
The cell membrane controls the passage of substances into and out of the cell. Inhibitors of the cell membrane function interfere with the cell membrane’s structure, causing cell contents to leak out, and eventually leading to cell death.
Daptomycin is a cell membrane inhibitor that is used to target Gram-positive bacteria. You may recall that Gram-positive cells have a large peptidoglycan cell wall, covering the plasma membrane. Daptomycin binds with calcium ions to form a calcium complex. The addition of the calcium to form the calcium complex means that the daptomycin can insert itself into the plasma membrane. The complexes then aggregate within the plasma membrane to form pore-like structures that allow ions to leak out of the cell, eventually leading to cell death (Figure 14). Daptomycin is a relatively new antibiotic and is not in widespread use, which may explain why resistance, for now, is rare.
Figure 14 Daptomycin (red) forms a complex with calcium which inserts into the cell membrane (blue) creating a pore like structure which allows ions (black) to leak out of the cell.
Show description|Hide description
This figure shows the mechanism of action of daptomycin. A section of the cell membrane is represented as a bilayer. The daptomycin molecule is shown on the outside of the membrane as a red circle with a long tail. In step 1, calcium binding and insertion, a daptomycin molecule is shown bound to calcium. The tail of this molecule is inserted into the membrane so that the tail sits within the membrane bilayer. The ‘head’ of the molecule sits on the outside surface of the membrane. In step 2, oligomerisation and channel formation, three calcium bound daptomycin molecules are shown inserted into the membrane (as in step 1) and clustered together. In step 3, ion efflux, a cluster of four calcium-bound daptomycin molecules is depicted as a channel, or pore, in the membrane. These molecules have their tails inserted into the membrane and the ‘head’ of the molecules sit on the outside surface of the membrane. There are potassium ions on either side of the channel made by the daptomycin molecules, on the inside and outside of the membrane. An arrow running through the channel indicates the direction of movement of the potassium ions from the inside of the membrane to the outside.
Figure 14 Daptomycin (red) forms a complex with calcium which inserts into the cell membrane (blue) creating a pore like structure which ...
Polymyxins, such as colistin, are another group of cell membrane inhibitors that are used to target Gram-negative cells. Remember, the Gram-negative cell has an additional outer membrane that is not present in Gram-positive cells. Polymyxins bind to the lipopolysaccharides within the outer membrane, causing structural changes to the membrane. This in turn causes a loss of membrane integrity, increasing its permeability. The polymyxins are now able to pass through the proteoglycan wall and reach the plasma membrane, disrupting it in a similar way. This causes the cell contents to leak out, eventually leading to cell death.
2 How do bacteria become resistant to antibiotics?
The ability to divide rapidly and reproduce exponentially means that bacteria are constantly mutating and adapting to survive any changes in their environment. Antibiotic resistance is the ability of bacteria to resist the action of antibiotics so that they survive exposure to antibiotics that would normally kill them or stop their growth (CDC, 2017; PHE, 2017).
You could be forgiven for thinking that antibiotic resistance is only caused by human use, and misuse, of antibiotics. However, as you will see in the following video, bacteria that have not been exposed to antibiotics used by humans have acquired resistance to many of the antibiotics we use to treat infections; as you will see later, some may be inherently resistant if they lack the pathway or component that the antibiotic acts on: for example, colistin has no effect on Gram-positive organisms, because they do not have an outer membrane. This video also appears in The problem of AMR, where it was included to show the ubiquity of antimicrobial resistant bacteria. We include it again here so that you can begin to think about the mechanisms that give rise to AMR, which you’ll learn about below.
Transcript: Video 2 Antibiotic resistance is a natural bacterial defence mechanism.
MICHAEL MOSLEY:
It was in 2012, in a cave much deeper than I'm able to go, that Hazel's team made their breakthrough discovery.
HAZEL:
So the kind of areas that we sample look quite a bit like this.
MICHAEL MOSLEY:
Hazel took a bunch of bacterial samples from the cave and sent them off to a lab for analysis. The results shocked everyone.
HAZEL:
So I sent him just 100, and he started testing them. And he's like, you're not going to believe this, but they're resistant to everything. Everything that's used.
MICHAEL MOSLEY:
So these were bacteria you found on a wall in a cave.
HAZEL:
Much, much more remote than this, much further way.
MICHAEL MOSLEY:
That had not seen humans for...
HAZEL:
We know humans had never been in there because we have the exploration records. So there was no impact on it. And they were resistant to practically every antibiotic that's used in the clinic.
MICHAEL MOSLEY:
That is both incredibly exciting and incredibly scary. Nobody had ever thought that you would find resistant bacteria down at the bottom of a bloody cave.
MICHAEL MOSLEY:
They’ve had no interaction with humans. But the bacteria Hazel found in the cave were resistant to a huge array of antibiotics we use in modern medicine. This resistance had clearly evolved over millions of years without us having anything to do with it. Why?
Well, it makes sense when you think of antibiotics not as man-made, but the byproduct of war between microbes. They make chemical weapons to destroy their enemies and steal their resources, weapons we have learned to exploit as antibiotics. The bacteria living deep in the cave have had millions of years to evolve weapons that can target and destroy even their toughest rivals. The battle for scarce resources, like nutrients and energy, is particularly brutal down here in the caves.
HAZEL:
It's really starved down here. There's no resources. It's probably one of the most starved environments on Earth. Because if you think about it, any energy needs to come in through the rock. And so there's a big competition for nutrients.
MICHAEL MOSLEY:
The fewer the resources, the more intensely the microbes battle. And that creates resistance. Because the bacteria under attack don't just lie back and die. When billions of bacterial cells are bombarded, all it takes is for one cell to mutate its DNA in such a way that the antibiotic can no longer kill it. This ability to resist then spreads fast. So developing resistance to antibiotics is an entirely natural process which the bacteria of Carlsbad have taken to the extreme.
End transcript: Video 2 Antibiotic resistance is a natural bacterial defence mechanism.
As you may now realise, many antibiotics in clinical use have originated from substances used by micro-organisms to defend themselves against other micro-organisms. In essence, humans have taken these important molecules and made use of them to produce the antibiotics we use. It therefore makes sense that bacteria would derive ways to defend themselves from antibiotics.
2.1 Antibiotic resistance mechanisms
Bacteria have evolved several sophisticated antibiotic resistance mechanisms. Figure 15 gives an overview of the major mechanisms by which bacteria become resistant to the action of antibiotics. Don’t worry if you don’t understand all of these terms, as they will be explained in the following sections.
Figure 15 An overview of the mechanisms of antibiotic resistance.
Show description|Hide description
This figure is a schematic diagram giving an overview of the mechanisms of antibiotic resistance. The bacterial cell is represented by a brown oval and the cell wall/membrane is represented in orange surrounding the bacterial cell. Antibiotics (shown as blue spheres) are shown (using red arrows) crossing the cell wall and binding to their target (in green). Enzymes that destroy or modify the antibiotic or target are shown in orange. The following mechanisms are illustrated; modifying the target, protecting the target, amplifying the target, preventing antibiotic entry, increasing efflux, modifying the antibiotic and destroying the antibiotic.
Figure 15 An overview of the mechanisms of antibiotic resistance.
In this section you will look at the three main mechanisms of antibiotic resistance:
modifying the antibiotic target
destroying or modifying the antibiotic
preventing the antibiotic from reaching its target.
Although you will look at each of these mechanisms in turn, it is worth remembering that bacteria may use multiple resistance strategies simultaneously to survive exposure to antibiotics.
2.1.1 Modifying the antibiotic target
As you already know, antibiotics are selectively toxic because they target structural features or cellular processes in the bacterial pathogen that are different or lacking in the host’s cells. Recall how penicillin and other related β-lactam antibiotics work by binding to penicillin-binding proteins (PBPs), preventing them from binding to their normal target, peptidoglycan. Or how trimethoprim prevents dihydrofolate reductase reacting with dihydrofolic acid.
Changes to the structure of the target that prevent efficient antibiotic binding but still enable the target to carry out its normal function will confer antibiotic resistance (Figure 16).
Figure 16 Schematic diagram showing how structural changes in a target enzyme can lead to antibiotic resistance. The substrate is the chemical on which the target enzyme acts. It binds to the enzyme and is converted into a product or products through the action of the enzyme.
Show description|Hide description
This schematic diagram shows an antibiotic-sensitive target protein molecule (in red). It has an antibiotic-binding site which is close to its substrate-binding site, so that when the antibiotic (in green) is bound, the substrate (in blue) is excluded. A mutation that alters the structure of the target protein such that it no longer has an antibiotic-binding site makes the target antibiotic-resistant as it can now bind its substrate in the presence of the antibiotic.
Figure 16 Schematic diagram showing how structural changes in a target enzyme can lead to antibiotic resistance. The substrate is the ...
This resistance strategy is widespread among bacteria. Altering the structure of the PBP, for example, is one way that bacteria develop resistance to penicillin.
2.1.2 Destroying or modifying the antibiotic molecule
The second mechanism of antibiotic resistance you will look at is the destruction or modification of the antibiotic by bacterial enzymes. Probably the most well studied example of enzymes that destroy antibiotics are the β-lactamases.
As you may recall, β-lactamases deactivate the β-lactam ring of β-lactam antibiotics, preventing them from binding to their target (Figure 17).
Figure 17 Inactivation of a β-lactam antibiotic by β-lactamase.
Show description|Hide description
This figure is a chemical equation showing the ring-opening reaction. The bond in the beta-lactam ring (highlighted in pink) that is broken is the amide linkage between the carbonyl (C=O) carbon and the nitrogen to which it is attached. This N atom is shared between the beta-lactam ring and the adjacent ring. The result of the hydrolysis reaction is a free carboxyl (COOH) group and an NH group in the remaining ring.
Figure 17 Inactivation of a β-lactam antibiotic by β-lactamase.
The first β-lactamase to be identified was penicillinase. As its name suggests, penicillinase can hydrolyse penicillin but not other, more recently discovered β-lactam antibiotics like cephalosporins. In the 1980s, a new group of β-lactamase enzymes were detected in Europe that hydrolyse a wider range of β-lactams. These enzymes are known as extended spectrum β-lactamase (ESBL).
ESBLs can deactivate almost all of the β-lactam antibiotics currently in therapeutic use. Consequently, their presence significantly reduces the available treatment options for infections caused by bacteria expressing β-lactamase. Since they were first described in the early 1980s, the frequency of infections caused by ESBL-producing bacteria has been increasing. Consequently, ESBLs represent an ever-growing healthcare challenge.
Some β-lactamases can be counteracted by using a β-lactamase inhibitor (a drug that can block the action of the enzyme, such as clavulanic acid or tazobactam) in combination with a β-lactam antibiotic (e.g. clavulanic acid and amoxicillin are combined in co-amoxiclav); however, even these are not necessarily effective against ESBLs, and each combination needs to be tested against the bacteria before using clinically.
How might a β-lactamase inhibitor help the treatment of infections caused by β-lactamase-expressing bacteria?
The β-lactamase inhibitor will block the ability of the β-lactamase to deactivate the β-lactam antibiotic so that the antibiotic can bind to its target molecule.
Other antibiotic-modifying enzymes do not destroy or target the core chemical structure that confers antibacterial activity. Instead they modify the antibiotic’s structure by adding chemical groups that prevent it from binding to its target. One group of antibiotics that are particularly susceptible to modification are the aminoglycoside antibiotics which include streptomycin and gentamicin (Figure 18).
Figure 18 Structure of streptomycin. An exposed hydroxyl (-OH) group that can be modified by aminoglycoside-modifying enzymes is highlighted in green (in the figure the hydroxyl group is shown as –HO – this is the same as –OH).
Show description|Hide description
This figure shows the structure of streptomycin, an aminoglycoside. It has three rings including both a 5-membered and a 6-membered sugar ring. All three rings have hydroxyl and amino groups attached; and one of the two OH substituents (highlighted in green) on the 6-membered sugar ring is a target site for inactivating enzymes.
Figure 18 Structure of streptomycin. An exposed hydroxyl (-OH) group that can be modified by aminoglycoside-modifying enzymes is ...
Aminoglycoside-modifying enzymes add bulky chemical groups to the exposed hydroxyl (–OH) and amino (–NH2) groups of the antibiotic, thus preventing it from binding to its target.
2.1.3 Preventing entry, increasing exit
Antibiotics are only effective if they can reach their target. Preventing antibiotics from reaching their target is the final mechanism of antibiotic resistance that you will look at in this module.
As you should recall, the cell wall protects bacteria from osmotic and mechanical damage. To reach their targets inside the cell, antibiotics must cross this cell wall. Bacteria can either modify their cell wall to prevent antibiotics entering the cell or can use special structures, called efflux pumps, to remove antibiotics that have gained entry. In Activity 5 you will look at these mechanisms in more detail.
Activity 5: Transporting antibiotics across the bacterial cell wall
Timing: Allow 15 minutes
First, watch the following animation which describes how antibiotics are transported across the bacterial cell wall.
Transcript: Video 3 Animation of the mechanisms of transport of antibiotics across the membrane.
INSTRUCTOR:
In this activity, you'll look at how altering the transport of antibiotics across the membrane can result in antibiotic resistance. The cell walls of Gram-positive bacteria are permeable to most antibiotics, represented here as blues spheres and triangles. Gram-positive bacteria are susceptible to these antibiotics because the antibiotic can cross the membrane and reach their targets, here shown in dark green inside the bacterial cell.
However, the outer membrane of Gram-negative bacteria, like E. coli, forms a permeability barrier that prevents antibiotics from entering the bacterial cell and reaching their target. To reach their target inside Gram-negative bacteria, antibiotics must overcome this permeability barrier. Embedded in the outer membrane of Gram-negative bacteria are proteins that form channels known as porins, shown here in light green. Antibiotics cross the outer membrane of Gram-negative bacteria by diffusing through these porin channels. Porin channels are fairly nonspecific and can transport many antibiotics across the membrane. The presence of porin channels in the outer membrane makes bacteria susceptible to antibiotics.
Some antibiotics can be efficiently removed from bacteria by efflux. Efflux is the movement of molecules out of the cell. Antibiotics are transported out of the bacterial cell by efflux pumps in the membrane, shown here in purple. Removing the antibiotic from the cell prevents it from binding to its target, so bacteria expressing efflux pumps are resistant to antibiotics. Some efflux pumps are specific and only transport one class of antibiotics, but many transport a wide range of molecules. These efflux pumps are known as multi-drug-resistant efflux pumps.
Porins and efflux pumps have opposite effects on the concentration of antibiotic inside the cell. In the following animations, the concentration of antibiotic inside the cell is shown by the brown colour. As the concentration of antibiotics increases, the colour becomes darker. The number of porins and efflux pumps on the outer membrane of bacteria can be altered, and these changes can affect the concentration of antibiotic inside the cell, and therefore, the susceptibility of bacteria to antibiotics. Watch what happens to the concentration of antibiotics as they enter the cell via porins and are removed by efflux pumps, and then answer the following questions.
End transcript: Video 3 Animation of the mechanisms of transport of antibiotics across the membrane.
(a) decreases the amount of antibiotic entering Gram-negative bacteria
b.
(b) increases the amount of antibiotic entering Gram-negative bacteria
c.
(c) has no effect on the amount of antibiotic entering Gram-negative bacteria.
The correct answer is a.
a.
Your answer is correct. Most antibiotics cannot cross the outer membrane of Gram-negative bacteria and therefore enter the cell via porin channels. Decreasing the number of porin channels will decrease the amount of antibiotic entering the bacteria.
a.
(a) very few porin channels on their outer membrane or have replaced their porin channels with channels that exclude fluroquinolone antibiotics
b.
(b) numerous porin channels on their outer membrane
c.
(c) replaced their porin channels with channels that selectively transport fluroquinolone antibiotics.
The correct answer is a.
a.
Your answer is correct. If an antibiotic cannot reach its target, bacteria will be resistant to its action. Decreasing the expression of porins, or replacing them with channels that cannot transport the antibiotic, will prevent the antibiotic from crossing the outer membrane and reaching its target; therefore, these bacteria will be resistant.
a.
(a) increase the amount of fluroquinolone antibiotics in the bacterial cell
b.
(b) decrease the amount of fluroquinolone antibiotics in the bacterial cell
c.
(c) have no effect on the amount of fluroquinolone antibiotics in the bacterial cell.
The correct answer is b.
b.
Your answer is correct. Efflux pumps actively transport antibiotics out of the bacterial cell. Therefore, increasing transport through these channels will decrease the amount of antibiotic inside the cell.
a.
(a) efflux pumps that are unable to transport fluroquinolone antibiotics
b.
(b) efflux pumps that transport fluroquinolone antibiotics
c.
(c) no efflux pumps.
The correct answer is b.
b.
Your answer is correct. If an antibiotic cannot reach its target, bacteria will be resistant to its action. Actively transporting antibiotics out of the cell decreases their concentration inside the cell, so that they cannot build up to a high enough concentration to exert the effect on their target.
Increasing active transport by expressing more efflux pumps that can actively transport the antibiotic out of the cell decreases the amount of antibiotic inside the cell and prevents it from acting on its target.
As you should now appreciate, bacteria can prevent antibiotics from reaching their target by decreasing the permeability of their outer membrane or by actively transporting antibiotics out of the cell (Activity 3). Both decreased porin expression and increased efflux pump expression have been reported in antibiotic-resistant clinical isolates. For example, P. aeruginosa strains that overexpress multidrug-resistant efflux pumps, which transport a wide range of antibiotics, have been isolated from patients (Kosmidis et al., 2012).
2.2 Intrinsic and acquired resistance
There are two ways in which bacteria can have these resistance mechanisms:
intrinsic (or inherent) resistance
acquired resistance.
In this section, you will look at each type in turn.
2.2.1 Intrinsic resistance
Intrinsic resistance is the innate ability of a bacterium to resist the action of an antibiotic as a consequence of the bacteria’s structural or functional characteristics. In contrast to acquired resistance, which you will look at next, intrinsic resistance is ‘normal’ for bacteria of a given type.
Intrinsic resistance may occur because bacteria lack the target for a particular antibiotic or because the drug can’t get to its target. It reduces the pool of antibiotics available to treat infections.
It is worth noting that resistance elements that are intrinsic to one bacterial type can occasionally be transferred to another one, and you will learn about this below.
2.2.2 Introducing acquired resistance
As its name suggests, acquired resistance is not innate to a bacterial type. It occurs when a bacterium acquires or develops the ability to resist the actions of a particular antibiotic.
Unlike intrinsic resistance, acquired resistance is only found in some populations of a bacterial type.
Acquired resistance is a very significant healthcare concern. Infections caused by bacteria that have acquired resistance to an antibiotic can no longer be treated with that antibiotic. Consequently, identifying the type of pathogenic bacteria causing an infection may not always be sufficient to determine which antibiotics will be effective treatments. That is why all isolates must be tested to determine which antibiotics are effective before definitive treatment can be prescribed.
Activity 6: Comparing intrinsic and acquired resistance
Timing: Allow about 10 minutes
Look at the following statements in the table. Decide whether they are about intrinsic or acquired resistance or both and type your answer into the right-hand column.
Active content not displayed. This content requires JavaScript to be enabled.
Intrinsic resistance, acquired resistance or both?
Mechanism only present in a subpopulation of bacteria of a given type
Acquired resistance
Can be identified if the bacterial type is known
Intrinsic resistance
Normal for bacteria of that type
Intrinsic resistance
Limits treatment options
Both
Mechanism present in all bacteria of a given type
Intrinsic resistance
Occurs as a result of genetic mutation or horizontal gene transfer
Acquired resistance
2.2.3 Multidrug resistance
Bacteria can acquire multiple resistance mechanisms, making them resistant to multiple antibiotics. This is known as multidrug resistance (MDR). MDR bacteria are often known as ‘superbugs’ (although it is important to note that being resistant does not make bacteria more pathogenic, just that it will be harder to treat) and they are a major concern because they severely limit available treatment options.
Multidrug resistance is increasing globally, both in human healthcare and in farm animals. The issues in human healthcare are obvious, but multi-drug resistance in the food chain could have a severe impact on our ability to treat animals, as well as consumers of affected meat, in the future.
Activity 7: Acquiring multidrug resistance
Timing: Allow 15 minutes
In this activity you are asked to read a short article and answer some questions about it. One article is relevant to animal health and the other to human health; select the article that is most appropriate for your job and your interests.
Animal health
Read the Medical News Today article ‘Antibiotic resistance in farm animals is rising fast’, which highlights that, although multidrug resistance is rare, it is increasing and could cause significant problems for treatment of both animals and humans in the future.
While you read the article, note down the answers to the following questions.
What percentage of antibiotics used in human treatment are routinely used in meat production?
Answer
73% are used in animals raised for food.
Which countries have the largest number of multidrug resistance cases among farm animals?
Answer
India and north-east China have the most cases. Kenya, Uruguay and Brazil follow close behind.
How much has the use of antibiotics in the last ten years affected resistance of bacteria found in samples from cattle, chickens and pigs?
Answer
The quantity of antimicrobials that bacteria found in cattle are resistant to has doubled. For chickens and pigs, the resistance is almost three times higher.
While you read the article, note down the answers to the following questions.
Which bacterium caused the patient’s infection?
Answer
The patient’s infection was caused by the Gram-negative bacterium Klebsiella pneumoniae.
How many antibiotics was the infection resistant to?
Answer
It was resistant to 26 different antibiotics, including the ‘drug of last resort’: colistin.
Is resistance to all antibiotics a common occurrence?
Answer
No. Infections that are resistant to all antibiotics are uncommon.
3 Why are so many bacteria resistant to antibiotics?
Acquired resistance can result from mutations in chromosomal DNA or through horizontal gene transfer. You will learn more about both these processes before considering why antibiotic resistance arises and spreads so rapidly.
3.1 How do mutations lead to resistance?
A bacterium can develop antibiotic resistance through genetic mutations that are permanent changes in the deoxyribonucleic acid (DNA) sequence that makes up a gene. Perhaps the best example of acquisition of resistance by mutation is Mycobacterium tuberculosis, where resistance to all therapeutic agents is caused by mutation.
How does altering the sequence of a bacterium’s DNA result in antibiotic resistance? The answer lies in how genetic information, encoded by DNA, is converted into proteins that are required for the structure and function of bacteria.
Optional activity: What do genes do?
If you are unfamiliar with the terms ‘DNA’, ‘RNA’, ‘base pair’, ‘gene’, ‘amino acid’ or ‘protein’, you may want to try our free OpenLearn course What do genes do? before you begin the following sections.
3.1.1 From genetic information to protein function
Almost every process in a cell requires proteins. As you saw previously, antibiotics often exert their bactericidal and bacteriostatic effects by binding to proteins that are crucial to the structure or function of the bacterial cell.
The function of a protein is largely determined by its structure. Proteins are composed of building blocks called amino acids. The sequence of these amino acids determines the structure of a protein. The amino acid sequence of a protein is specified by the DNA sequence of a gene (Figure 19). So, there is a direct relationship between DNA and the structure and function of a protein.
Figure 19 The DNA sequence of a gene encodes the sequence of amino acids in a protein.
Show description|Hide description
This figure is a schematic diagram showing three genes 1, 2 and 3, at different locations within the genome. Each gene is expressed (via transcription and translation) as a chain of amino acids in a specific sequence – chains 1, 2 and 3 respectively. Each amino acid chain then folds up into a particular three-dimensional structure, determined by its amino acid sequence.
Figure 19 The DNA sequence of a gene encodes the sequence of amino acids in a protein.
In 1958, Francis Crick, who helped discover the structure of DNA, proposed an explanation regarding how genetic information, encoded in DNA, can be converted into a functional product, a protein. In the first phase, a strand of DNA is used as a template to produce a complementary strand of RNA (ribonucleic acid). This process is called transcription. The RNA strand is then translated into a sequence of amino acids to form a new protein.
3.1.2 Genetic mutations and protein structure
As you already know, changes in the structure of bacterial proteins can result in antibiotic resistance.
Can you think of a specific example of how changing protein structure could lead to antibiotic resistance?
Structural changes to an antibiotic target protein could prevent that antibiotic from binding. This would make the target insensitive to the antibiotic and bacteria containing this protein would be resistant to the effects of that antibiotic. For example, linezolid exerts its antibiotic effects by binding to ribosomes and preventing the initiation of protein synthesis. Structural changes to the ribosome can prevent the binding of linezolid. Consequently, protein synthesis initiation is no longer blocked in the presence of linezolid and these resistant bacteria can grow.
Remember that the amino acid sequence, and therefore the structure of a protein, is encoded in the DNA sequence of a gene. Small changes, or mutations, in the DNA sequence within a gene can alter the amino acid sequence of the protein it encodes (Figure 20).
Figure 20 Genetic mutations can alter the amino acid sequence of a protein.
Show description|Hide description
Part (a) shows the normal sequence of DNA within a gene. This sequence codes for a sequence of amino acids within a protein. Each amino acid is represented as a blue block on a chain. Part (b) shows a mutation in the DNA sequence from part (a). This change is highlighted in orange. The amino acid sequence coded by this DNA sequence is shown. Amino acids are shown as blue blocks on a chain. The amino acid altered by the mutation is shown in brown.
Figure 20 Genetic mutations can alter the amino acid sequence of a protein.
Some changes in the bacterium’s DNA sequence have no effect on the amino acid sequence. Sometimes, however, even though the change in DNA is very small – perhaps a single point mutation – there can be a major effect on the amino acid sequence and, therefore, the structure of proteins that are targeted by antibiotics. As you have seen, these changes in the structure of proteins targeted by antibiotics can have important consequences for their function.
3.1.3 Transmission of mutations by vertical gene transfer
Vertical gene transfer is the transfer of genetic information, including any genetic mutations, from a parent to its offspring. Bacteria reproduce by binary fission, where the cell grows to twice its original size and then divides into two identical daughter cells. As in humans, the genetic information in bacteria is encoded in DNA, which is packed into chromosomes. During binary fission, the chromosomal DNA is copied, so that each new daughter cell inherits an exact copy of the parent cell’s chromosomes (Figure 21).
Figure 21 The stages of binary fission.
Show description|Hide description
This figure is a schematic diagram showing the process of binary fission. The bacteria cell is represented as a blue oval surrounded by a blue plasma membrane and a brown cell wall. The chromosomal DNA is represented as a purple squiggle inside the bacterial cell. The process begins with duplication of the chromosomes, represented by an increase in the amount of purple. This DNA is then separated, represented by the presence of two purple squiggles. During this process the cell grows, represented by elongation of the oval. The cell divides giving two cells that are identical to the first image.
Figure 21 The stages of binary fission.
Activity 8: Exploring vertical transmission
Timing: Allow about 10 minutes
Apply what you have learned to complete the following sentences. Select the appropriate word from the list.
a.
identical
b.
similar
c.
different
The correct answer is a.
Answer
During binary fission, the genetic material (DNA) is copied so that each new daughter cell inherits an exact copy of the parent cell's DNA.
a.
sometimes
b.
never
c.
always
The correct answer is c.
Answer
During binary fission, the genetic material (DNA) is copied, so that each new daughter cell inherits an exact copy of the parent cell’s DNA. When the parent DNA is copied during binary fission, any genetic mutations will also be copied, and consequently inherited, by both of the daughter cells.
a.
sometimes
b.
always
c.
never
The correct answer is b.
Answer
During binary fission, the genetic material (DNA) is copied, so that each new daughter cell inherits an exact copy of the parent cell’s DNA. When the parent DNA is copied during binary fission, any genetic mutations will also be copied, and consequently inherited, by both of the daughter cells. If these genetic mutations give rise to antibiotic resistance in the parent bacteria, they will also result in antibiotic resistance in both of the daughters.
Vertical gene transfer is only one of the ways in which bacteria can spread antibiotic resistance genes. In the next section you will look at another – horizontal transfer.
3.2 Horizontal transfer
Horizontal gene transfer, or horizontal transfer, is the primary mechanism of spread of antibiotic resistance that allows bacteria to spread antibiotic resistance genes rapidly between different bacteria, and can sometimes include transfer to different bacterial species. As you should be starting to appreciate, the acquisition of antibiotic resistance by new bacterial types is particularly concerning because it can result in multidrug-resistant bacterial strains such as MRSA.
Figure 22 Antibiotic resistance.
Show description|Hide description
Antibiotic resistance cartoon. The cartoon features two bacteria, one is dressed in a coat and mac and hiding around a corner. This bacteria is holding a double helix of DNA. The speech bubble says ‘pssst! Hey kid! Wanna be a superbug? Stick some of the into your genome… even penicillin won’t be able to harm you!’ The caption reads ‘It was on a short-cut through the hospital kitchens that Albert was first approached by a member of the Antibiotic Resistance.’
Figure 22 Antibiotic resistance.
3.2.1 Plasmids
You have seen how chromosomal DNA can be copied and transmitted to the next generation via vertical gene transfer. Unlike humans and other animals, bacteria contain additional, non-chromosomal DNA which can be replicated independently of the genomic chromosomal DNA. These non-chromosomal genetic elements are called plasmids.
A plasmid is a small, circular piece of DNA that often carries genes associated with a specific function: for example, antibiotic resistance (Figure 23).
Figure 23 A simple plasmid containing one antibiotic resistance gene (ampR) and an origin of replication (colE1 origin); where DNA replication begins when the plasmid is replicated.
Show description|Hide description
This figure shows a schematic representation of a simple plasmid. The plasmid is represented as a blue circle. There are several blue arrows and boxes on the circle labelled ampR, colE1 origin, MCS and lacZ. There is a marker labelled BamHI, EcoRI pointing to the box labelled MCS.
Figure 23 A simple plasmid containing one antibiotic resistance gene (ampR) and an origin of replication (colE1 origin); where DNA ...
Plasmids are often transferred by horizontal gene transfer. This is the process of transferring genetic information between two unrelated cells. In contrast to vertical gene transmission, where plasmids are transferred from parents to daughter cells, it does not require binary fission and can occur between bacteria of the same generation, and even between bacteria of different species (Figure 24).
Figure 24 The differences between horizontal and vertical gene transmission.
Show description|Hide description
This figure is a schematic representation of the differences between horizontal and vertical gene transmission. On the left of the figure is a blue oval representing a bacterial cell. This cell contains a blue wavy line labelled chromosomal DNA and a red circle labelled resistance plasmid. On the blue wavy line there is a small orange region labelled resistance gene. There is an arrow going from the blue bacterial cell to a green oval representing another bacterial cell. The arrow is labelled horizontal transfer. The green cell contains a green wavy line labelled chromosomal DNA and a red circle labelled resistance plasmid. On the green wavy line there is a small red region labelled resistance gene. Below this cell is another cell which is identical except that the chromosomal DNA is duplicated. The image below this is of an elongated, squashed green cell containing two copies of the chromosomal DNA and one resistance plasmid. The final image is of two green cells. One cell contains only chromosomal DNA, the other cell contains chromosomal DNA and the resistance plasmid. There is a vertical arrow running from the first green cell at the top to the two green cells at the bottom. It is labelled vertical transfer.
Figure 24 The differences between horizontal and vertical gene transmission.
Can you suggest why horizontal gene transfer is the primary mechanism of spreading antibiotic resistance?
Horizontal gene transfer allows plasmids carrying antibiotic resistance genes to spread rapidly between different types of bacteria. This means that species of bacteria that are susceptible to a given antibiotic rapidly acquire resistance genes, making them resistant to treatment with that antibiotic.
There are three mechanisms of horizontal gene transfer:
conjugation
transformation
transduction.
You will now look at each mechanism in more detail.
3.2.2 Conjugation
In the process of conjugation, plasmids are transferred between two contacting bacteria via a hollow tube or pilus (Figure 25).
Figure 25 The process of conjugation. (a) A hollow pilus connects two bacteria and plasmid DNA is transferred from the donor bacterium to the recipient. (b) Scanning-electron micrograph of two bacteria attached by pili.
Show description|Hide description
Part (a) comprises a schematic representation of conjugation. Bacterial cells in blue are represented by a blue oval surrounded by a brown cell wall. Chromosomal DNA is represented as a purple squiggle. Plasmid DNA is represented as a green ring. The first part of the diagram shows two separate bacterial cells labelled donor and recipient. Both cells contain chromosomal DNA but only the donor contains the plasmid. In the next image the donor and recipient cells are connected by a tube which is coloured blue and surrounded by a brown cell wall. The tube is labelled pilus. The next images shows the plasmid DNA passing through the pilus. The final image shows the two bacterial cells still linked via a pilus however now both the donor and recipient cells contain plasmid DNA. Part (b) shows a scanning electron micrograph of two orange bacterial cells on a dark background. The cells are joined by two orange pili.
Figure 25 The process of conjugation. (a) A hollow pilus connects two bacteria and plasmid DNA is transferred from the donor bacterium to...
Since antibiotic resistance genes are often located on plasmids, conjugation can result in the transfer of antibiotic resistance from one bacterium to another.
3.2.3 Transformation
In contrast to conjugation, the process of transformation allows bacteria to take up DNA from their environment (for example, from a lysed bacterium) across the cell wall. This DNA can then be incorporated into the genome of the bacterium (Figure 26).
Figure 26 Schematic diagram of a bacterium taking up DNA from the environment by transformation.
Show description|Hide description
This figure shows a schematic representation of the process of transformation. Bacterial cells are represented as before by a blue oval surrounded by a brown cell wall. Bacterial chromosomal DNA is represented as a purple circle. DNA fragments are represented as pink lines. In the first image DNA fragments appear stuck to the outside of the bacterial cell. There is an arrow to the next image which shows a bacterial cell containing one DNA fragment. There is an arrow from this cell to a final bacterial cell in which the pink DNA fragment is incorporated into the purple chromosomal DNA.
Figure 26 Schematic diagram of a bacterium taking up DNA from the environment by transformation.
Transformation occurs naturally between some bacteria, such as Streptococcus pneumoniae and Haemophilus influenza. When antibiotic resistance genes in the environment are transformed into a new bacterial type, they can be incorporated into that bacterium’s genome. They are then transmitted to the next generation by binary fission, establishing a newly resistant population of bacteria. Unincorporated plasmids are also usually (but not always) passed to the next generation.
3.2.4 Transduction
The final mechanism of horizontal gene transfer you will look at is transduction. In this process, transfer of DNA from one bacterial cell to another is mediated by a virus.
Viruses that infect bacteria are called bacteriophages. When a bacteriophage infects a bacterial cell, it inserts its DNA into the bacterial cell genome. When it is time for the virus to replicate, it excises its DNA from the bacterial genome. However, this excision is imperfect and some bacterial DNA may be accidentally excised and incorporated into the newly made virus. When these newly made viruses infect a different bacterial species, they carry this bacterial DNA, which may contain antibiotic resistance genes, and insert it into the genome of the new host bacterium (Figure 27).
Figure 27 Schematic diagram of transduction. When bacteriophage DNA, shown by a dotted line, is excised from the bacterial genome it carries with it some bacterial DNA, shown as a solid line, from the infected bacteria. (You should view the larger version of this image to see the dotted line clearly.) this DNA is incorporated into new bacteriophage particles which are released and infect new bacteria of a different species. The bacterial DNA from the original bacteria is incorporated into the genome of the newly infected bacteria.
Show description|Hide description
This figure is a schematic representation of transduction. Bacterial cells of one species are represented as blue ovals. The bacterial DNA is represented as a blue circle. Bacteriophage DNA is represented in a black dotted line. Bacteria infected with bacteriophage have bacteriophage DNA incorporated into their DNA. When this DNA is excised from the bacterial genome if carries with it some bacterial DNA. This DNA is incorporated into new bacteriophage particles which are released and infect new bacteria of a different species (in green). The bacterial DNA from the original bacteria, in blue, is incorporated, together with the bacteriophage DNA (in red) into the genome of the newly infected bacteria (in green).
Figure 27 Schematic diagram of transduction. When bacteriophage DNA, shown by a dotted line, is excised from the bacterial genome it ...
Activity 9: Comparing horizontal transfer mechanisms
Timing: Allow about 5 minutes
Use the drop-down options to complete the five sentences below.
Active content not displayed. This content requires JavaScript to be enabled.
Within a mixed bacterial population (for example, in the gastrointestinal system), some bacteria may be susceptible to antibiotic treatment while others are intrinsically resistant, or may have acquired resistance to antibiotics, via either genetic mutation or horizontal gene transfer. In the presence of antibiotics, the resistant bacteria have a survival advantage over the susceptible bacteria and are more likely to survive and reproduce. Because bacteria reproduce so quickly, resistant bacteria can quickly dominate the population (Figure 28).
Figure 28 Natural selection for antibiotic resistance. When susceptible bacteria, shown in blue, are treated with antibiotics they all die. However, antibiotic-resistant bacteria (shown in green) survive and replicate by binary fission. This new bacterial population is now completely non-susceptible to treatment with the same antibiotic.
Show description|Hide description
This figure shows a schematic representation of how natural selection selects for antibiotic-resistant bacteria. The left hand side of the figure shows four susceptible bacteria cells represented as blue ovals. Following antibiotic treatment these cells die (represented as transparent blue ovals). The right hand side of the figure shows a population of bacteria in which one cell (in green) is resistant to antibiotics. Following treatment susceptible cells die however the resistant cell survives and replicates leading to a population of resistant (green cells) that do not die following antibiotic treatment.
Figure 28 Natural selection for antibiotic resistance. When susceptible bacteria, shown in blue, are treated with antibiotics they all ...
Of course, changes to the bacteria’s environment made by us can affect the evolution of antibiotic resistance.
3.3.1 Case study – antibiotics relevant to your work
Throughout this module you have studied the mechanism of action of several classes of antibiotics. In Activities 10–12 you will use an online database and apply what you have learnt to find out about the mechanism of action of an antibiotic related to your work.
In the following activity you will use the online Comprehensive Antibiotic Resistance Database (CARD) to find out some basic information about the mechanism of action of antibiotics relevant to your work.
Activity 10: Identifying antibiotics relevant to your work
Timing: Allow about 15 minutes
You are asked to read a short paragraph and answer some questions about it. One paragraph is relevant to human health and the other to animal health. Select the paragraph that is most appropriate for your job and your interests. Identify three antibiotics that are relevant to your work.
Animal health
Mastitis is extremely common in dairy cattle, and is very costly for farmers. It is usually an infection of the mammary gland that can be caused by physical injury, but the most common cause is environmental contamination. Antibiotics available to treat bacterial mastitis include sulphonamides (an example of which is sulphanilamide), penicillin and streptomycin.
You can either use the mastitis example above or choose another disease from your work and the related antibiotics used to treat it. You may wish to talk to a colleague or supervisor to find out what diseases are commonly encountered in your workplace for ideas.
Use the space below to make a note of any antibiotics used in your work.
By signing in and enrolling on this course you can view and complete all activities within the course, track your progress in My OpenLearn Create. and when you have completed a course, you can download and print a free Statement of Participation - which you can use to demonstrate your learning.
Type the name of one of the antibiotics that you identified in Activity 10 into the search box on the top right of the CARD website (highlighted in Figure 29).
Figure 29 Screenshot of the CARD database. The search box on the top right is highlighted.
Select the antibiotic name from the resulting drop-down list (see Figure 30).
Figure 30 Screenshot from the CARD database showing the drop-down search menu.
This should open a webpage containing information about the antibiotic. An example is shown in Figure 31.
Figure 31 Screenshot of an antibiotic page in the CARD database. The drug class link is highlighted.
Explore the entry for the antibiotic that you have selected. Note down in the space below any information you can find about:
the drug class that the antibiotic belongs to
whether its action is bacteriostatic or bactericidal
whether it is active against Gram-negative or Gram-positive bacteria, or both
whether it is broad- or narrow-spectrum
the bacterial process that it interferes with.
Be aware that some information about the antibiotic may be given on the drug class page. To access this information you will need to click on the name of the drug class to open the link (highlighted in Figure 31).
Do not worry if you cannot find all of this information in the CARD database. You could also try searching online or asking your colleagues for information. Now repeat the process for two other antibiotics used to treat your chosen disease or pathogen.
To use this interactive functionality a free OU account is required. Sign in or register.
Write a short summary (of four or five sentences) about what you have found out. Include the name of the antibiotic and why you have selected it. To help you, an example of a forum post is given below:
Animal health example
Bacterial diarrhoea, or scour, in piglets can be treated using cephalexin as long as the bacterial pathogen is susceptible to it. In my workplace we carry out regular tests to determine whether diarrhoeal samples from pigs from the local farms are resistant to cephalexin. Cephalexin is a cephalosporin antibiotic. It is a broad-spectrum, bactericidal antibiotic which acts by inhibiting bacterial cell wall synthesis. It is active against both Gram-positive and Gram-negative bacteria.
Human health example
In my workplace we carry out regular tests to determine whether diarrhoeal samples from the local community are resistant to cephalexin. Cephalexin is a cephalosporin antibiotic. It is a broad-spectrum, bactericidal antibiotic which acts by inhibiting bacterial cell wall synthesis. It is active against both Gram-positive and Gram-negative bacteria.
Once you have written your three summaries, spend a few minutes comparing the different antibiotics and how they act. Are they from different classes, or the same? Do they have the same mechanism of action or different? What else is similar or different about them?
To use this interactive functionality a free OU account is required. Sign in or register.
Open the quiz in a new tab or window by holding down ‘Ctrl’ (or ‘Cmd’ on a Mac) when you click on the link.
5 Summary
In this module you looked at the main modes of antibiotic action and learned why these drugs work selectively against bacteria without causing the same level of harm to the host. You have also learned that bacteria can be intrinsically resistant to antibiotics or acquire antibiotic resistance that can rapidly spread, or evolve, in a population. You should now be able to explain how genetic mutations cause acquired antibiotic resistance and how these mutations can be inherited through binary fission.
You have also seen how horizontal gene transfer has an important role in transmitting antibiotic resistance to different bacterial types. Having seen how antibiotic resistance evolves to protect bacteria, you can now begin to speculate on how our use of antibiotics contributes to the rise of antibiotic resistance.
You should now be able to:
explain how antibiotics work against bacterial pathogens
state what is meant by the term ‘antibiotic resistance’
explain that antibiotic resistance is a natural phenomenon that protects bacteria from hostile environments
explain how resistance mechanisms arise, and how resistance can be passed on during bacterial replication
describe the horizontal gene transfer mechanisms that allow antibiotic resistance to be transferred between bacteria
apply scientific terminology when explaining how antibiotic resistance relates to your current work.
Now that you have completed this module, consider the following questions:
What is the single most important lesson that you have taken away from this module?
How relevant is it to your work?
Can you suggest ways in which this new knowledge can benefit your practice?
When you have reflected on these, go to your reflective blog and note down your thoughts.
Activity 11: Reflecting on your progress
Timing: Allow about 15 minutes
Do you remember at the beginning of this module you were asked to take a moment to think about these learning outcomes and how confident you felt about your knowledge and skills in these areas?
Now that you have completed this module, take some time to reflect on your progress and use the interactive tool to rate your confidence in these areas using the following scale:
5 Very confident
4 Confident
3 Neither confident or not confident
2 Not very confident
1 Not at all confident
Try to use the full range of ratings shown above to rate yourself:
Active content not displayed. This content requires JavaScript to be enabled.
Reflect on your progress by comparing your answers here to those you wrote at the beginning of the module. Have your responses changed?
Remember in Activity 2 where you were asked to construct your own glossary of terms? Has this helped to expand your understanding of your scientific knowledge relating to your own working environment?
When you have reflected on your answers and your progress on this module, go to your reflective blog and note down your thoughts.
6 Your experience of this module
Now that you have completed this module, take a few moments to reflect on your experience of working through it. Please complete a survey to tell us about your reflections. Your responses will allow us to gauge how useful you have found this module and how effectively you have engaged with the content. We will also use your feedback on this pathway to better inform the design of future online experiences for our learners.
Baym, M., Lieberman, T., Kelsic, E., Chait, R., Gross, R., Yelin, I. and Kishony, R. (2016) ‘Spatiotemporal microbial evolution on antibiotic landscapes’, Science, 3539(6304), pp. 1147–51 [online]. Available at https://science.sciencemag.org/content/353/6304/1147 (accessed 4 June 2020).
Bonnet, R. (2004) ‘Growing group of extended-spectrum β-lactamases: the CTX-M enzymes’, Antimicrobial Agents and Chemotherapy, 48(1), pp. 1–14 [online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC310187/ (accessed 4 June 2020).
Cartelle, M., del Mar Tomas, M., Molina, F., Moure, R., Villanueva, R. and Bou, G. (2004) ‘High-level resistance to ceftazidime conferred by a novel enzyme, CTX-M-32, derived from CTX-M-1 through a single Asp240-Gly substitution’, Antimicrobial Agents and Chemotherapy, 48(6), pp. 2308–13 [online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC415568/pdf/0618-03.pdf (accessed 4 June 2020).
Chen, Y., Delmas, J., Sirot, J., Shoichet, B. and Bonnet, R. (2005) ‘Atomic resolution structures of CTX-M β-lactamases: extended spectrum activities from increased mobility and decreased stability’, Journal of Molecular Biology, 348(2), pp. 349–62 [online]. Available at https://www.sciencedirect.com/science/article/abs/pii/S0022283605001634 (accessed 4 June 2020).
Cox, G. and Wright, G. (2013) ‘Intrinsic antibiotic resistance: mechanisms, origins, challenges and solutions’, International Journal of Medical Microbiology, 303(6–7), pp. 287–92 [online]. Available at https://www.sciencedirect.com/science/article/abs/pii/S1438422113000246 (accessed 4 June 2020).
Duplessis, C. and Crum-Cianflone, N. F. (2011) ‘Ceftaroline: a new cephalosporin with activity against methicillin-resistant Staphylococcus aureus (MRSA)’, Clinical Medicine Reviews in Therapeutics, 3, pp. 24–66 [online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3140339/ (accessed 4 June 2020).
Falgenhauer, M., Yao, Y., Fritzenwanker, M., Schmiedel, J., Imirzalioglu, C., Chakraborty, T. (2014) ‘Complete genome sequence of phage-like plasmid pECOH89, encoding CTX-M-15’, Genome Announcements, 2(2), pp. e00356–14 [online]. Available at https://mra.asm.org/content/2/2/e00356-14.short (accessed 4 June 2020).
Gallagher, J. (2017) ‘Bug resistant to all antibiotics kills woman’, BBC News, 13 January [online]. Available at https://www.bbc.co.uk/news/health-38609553 (accessed 5 June 2020).
Hernández-Allés, S., Conejo, M., Pascual, A., Tomás, J., Benedí, V. and Martínez-Martínez, L. (2000) ‘Relationship between outer membrane alterations and susceptibility to antimicrobial agents in isogenic strains of Klebsiella pneumoniae’, Journal of Antimicrobial Chemotherapy, 46(2), pp. 273–7 [online]. Available at https://academic.oup.com/jac/article/46/2/273/881421 (accessed 4 June 2020).
Humeniuk, C., Arlet, G., Gautier, V., Grimont, P., Labia, R. and Philippon, A. (2002) ‘β-Lactamases of Kluyvera ascorbata: probable progenitors of some plasmid-encoded CTX-M types’, Antimicrobial Agents and Chemotherapy, 46(9), pp. 3045–9 [online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC127423/ (accessed 4 June 2020).
Kisgen, J and Whitney, D. (2008) ‘Ceftobiprole, a broad-spectrum cephalosporin with activity against methicillin-resistant Staphylococcus aureus (MRSA)’, Pharmacy and Therapeutics, 33(11), pp. 631–41 [online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2730812/ (accessed 4 June 2020).
Kosmidis, C. Schindler, B., Jacinto, P., Patel, D., Bains, K., Seo, S. and Kaatz, G. (2012) ‘Expression of multidrug resistance efflux pump genes in clinical and environmental isolates of Staphylococcus aureus’, International Journal of Antimicrobial Agents, 40(3), pp. 204–9 [online]. Available at https://www.sciencedirect.com/journal/international-journal-of-antimicrobial-agents (accessed 27 November 2020).
Lim, D. and Strynadka, N. C. J. (2002) ‘Structural basis for the beta lactam resistance of PBP2a from methicillin-resistant Staphylococcus aureus’, Nature Structural Biology, 9(11), pp. 870–6 [online]. Available at https://www.nature.com/articles/nsb858 (accessed 4 June 2020).
Long, K. S., Poehlsgaard, J., Kehrenberg, C., Schwarz, S. and Vester, B. (2006) ‘The Cfr rRNA methyltransferase confers resistance to phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramin A antibiotics’, Antimicrobial Agents and Chemotherapy, 50(7), pp. 2500–5 [online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1489768/ (accessed 4 June 2020).
Pfizer (2017) Antimicrobial Testing Leadership and Surveillance (ATLAS) [online]. Available at https://atlas-surveillance.com/ (accessed 4 June 2020).
Potron, A., Nordmann, P., Rondinaud, E., Jaureguy, F. and Poirel, L. (2013) ‘A mosaic transposon encoding OXA-48 and CTX-M-15: towards pan-resistance’, Journal of Antimicrobial Chemotherapy, 68(2), pp. 476–7 [online]. Available at https://academic.oup.com/jac/article/68/2/476/674526 (accessed 4 June 2020).
Smet, A., Van Nieuwerburgh, F., Vandekerckhove, T. T. M., Martel, A., Deforce, D., Butaye, P. and Haesebrouck, F. (2010) ‘Complete nucleotide sequence of CTX-M-15-plasmids from clinical Escherichia coli isolates: insertional events of transposons and insertion sequences’, PLoS ONE, 5(6), p. e11202 [online]. Available at https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0011202 (accessed 4 June 2020).
Valent, P., Groner, B., Schumacher, U., Superti-Furga, G., Busslinger, M., Kralovics, R., Zielinski, C., Penninger, J. M., Kerjaschki, D., Stingl, G., Smolen, J. S., Valenta, R., Lassmann, H., Kovar, H., Jäger, U., Kornek, G., Müller, M. and Sörgel, F. (2016) ‘Paul Ehrlich (1854–1915) and his contributions to the foundation and birth of translational medicine’, Journal of Innate Immunity, 8, pp. 111–20 [online]. Available at https://www.karger.com/Article/Fulltext/443526 (accessed 4 June 2020).
This free course was written by Kay Saunders and Rachel McMullan, with contributions from Ben Amos, Claire Gordon, Natalie Moyen and Hilary MacQueen.
Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.
The material acknowledged below is Proprietary and used under licence (not subject to Creative Commons Licence). Grateful acknowledgement is made to the following sources for permission to reproduce material in this free course:
Video 2: BBC, ‘Michael Mosley vs The Superbugs’, TX 17 May 2017.
Video 3: The Open University.
Every effort has been made to contact copyright owners. If any have been inadvertently overlooked, the publishers will be pleased to make the necessary arrangements at the first opportunity.