1.2 Potential bacterial targets for antibiotics

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. 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. 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. 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: 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. Explain why antibiotics that target cell wall synthesis leave eukaryotic cells unharmed. Answer: Eukaryotic cells lack a cell wall.

Penicillin's a bactericidal. Penicillin's 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.