The Open University since 2006
Alternatively you can skip the navigation by pressing 'Enter'.
Exodus: Our Journey To Europe: EPISODE 1Wednesday, 27th July 2016 02:45 - BBC TwoEpisode 1 begins as hundreds of thousands of refugees from Syria arrive in the Turkish port of Izmir. Read more: Exodus: Our Journey To Europe: EPISODE 1
Exodus: Our Journey To Europe: EPISODE 1Available until Friday, 26th August 2016 03:45Episode 1 begins as hundreds of thousands of refugees from Syria arrive in the Turkish port of Izmir. Read more: Exodus: Our Journey To Europe: EPISODE 1
Discovery: The Truth About Success And Failure In Medicine: Going Lean: Heath and the Toyota WayAvailable for over a year
Thinking Allowed July 2016: Food bank Britain, Food poverty in EuropeAvailable for over a year
Full Steam Ahead: Episode oneAvailable until Monday, 22nd August 2016 17:30
Percy Shelley: PolemicistThe political writing of Percy Shelley might have a message for the UK right now, believes Mark... Read more: Percy Shelley: Polemicist
Full Steam AheadIt’s Full Steam Ahead for historians Ruth Goodman, Alex Langlands and Peter Ginn as they bring... Read more: Full Steam Ahead
Language and creativityThis free course is an introduction to the relationship between language and creativity, to the... Try: Language and creativity now
An introduction to music theoryGain an understanding of the basic building blocks of musical theory and notation. This free... Try: An introduction to music theory now
Microbes often get a bad name. Whilst some of them do cause disease, others play vital roles in recycling nutrients in the soil to enable plants to grow, and in breaking down human waste. Without microbes, we would have no beer, no yoghurt, no coffee. That's quite impressive for something too small to see. This free course, Microbes friend or foe? sheds some light on them.
After studying this course, you should be able to:
- demonstrate general knowledge and understanding of some of the basic facts, language, concepts and principles relating to microbiology and its importance and applications in biological and heath sciences and environmental sciences
- understand the contribution that microbiology can make to informed debate on medical and environmental issues
- make sense of information presented in different ways, including textual, numerical and graphical material.
Study this free course
Enrol to access the full course, get recognition for the skills you learn, track your progress and on completion gain a statement of participation to demonstrate your learning to others. Make your learning visible!
1 Disease-causing bacteria
Surprisingly, many bacteria that cause disease are ones that are naturally present in or on our bodies. Bacteria that co-exist with the human body are known as commensal bacteria. It is a change in the normal situation, caused for example by a wound, which gives them a chance to grow and become a menace rather than remain a harmless natural inhabitant. Commensals also maintain a natural microbial balance in the body and help prevent pathogens from becoming abundant.
One of the most important groups of commensal bacteria is the genus Streptococcus, which can cause numerous infections including pneumonia. Streptococcus pyogenes is perhaps the most infamous and is sometimes known as the ‘flesh-eating’ bacterium. In fact, the condition known as necrotising fasciitis, an infection of the deeper layers of the skin, causing large-scale deterioration of flesh, is very rare. More commonly Streptococcus pyogenes strains are involved in causing skin infections, meningitis and pharyngitis (‘strep throat’); each one of these may be caused by different strains. About one-tenth of the population carry the Strep. pyogenes bacterium around in their throats and noses and usually it does no harm at all. (Note that because the genera Streptococcus and Staphylococcus can be easily confused if referred to in their abbreviated form, simply by the first letter of their genus, their names are instead often shortened to Strep. and Staph.) Strep. pyogenes is not destroyed by the immune system because it carries proteins on its cell surface that resemble human proteins, thus fooling the system into thinking it is part of the human body and preventing it being destroyed. About one-fifth of the population have Strep. pneumoniae in their body and it too can produce a surface layer that allows it to evade the immune system. A commensal species can cause an infection if its growth becomes out of balance with the rest of the natural microbial population in the human body. The population size and activity of each commensal species are normally kept in check by competition from other commensals, but if the balance is disrupted then some commensals or even foreign microbes may get a chance to multiply and become more active.
Many of the anaerobic bacteria naturally found in our gut may also become the cause of disease. The most common culprits are those in the phylum Bacteroides, which most of the time are beneficial. They exclude more dangerous pathogens from the gut, essentially by out-competing them for space and food. However, outside the gut and out of control in other areas of the body, these organisms can become a serious problem, causing abscesses, blood infection, infections under the skin and infections of the abdominal cavity.
Bacteria of the genus Clostridium are common in soil and can be responsible for a range of diseases in people. Often they are picked up from soil or from poorly cleaned food.
Which species of Clostridium causes tetanus?
Tetanus is caused by C. tetani.
The clostridia produce toxins, which affect the nervous system. So, if C. tetani gets into a wound and multiplies, the toxins are released, enter the bloodstream and from there travel to the nerves and result in the excessive contraction of some of the muscles in the face, causing lockjaw (the common name for tetanus). Of all the clostridia species in the wild, C. botulinum produces the most powerful toxin and it is responsible for botulism, a type of food poisoning that is potentially fatal. The best known of the clostridia is C. difficile, one of the few clostridia naturally present in the human population. About 3% of adults are thought to carry it in their gut. It rarely causes disease unless the natural population of microbes in the gut has been disturbed, such as during the administering of antibiotics, which kill off many of the natural gut microbes. In this case, C. difficile can multiply uncontrollably and release its toxins causing toxic shock syndrome (TSS), with a range of symptoms including a rash, high fever, low blood pressure, confusion and ultimately coma and multiple organ failure, resulting in death.
Not all bacteria produce spores, but C. difficile does. Why might this be a problem in treating it?
The spores produced by the bacterium allow it to be spread around easily in the environment, including amongst staff and patients in hospitals. The spores also give C. difficile resistance to many extreme environmental conditions, which allows it to survive for long periods of time in the natural environment outside human bodies and makes it difficult to kill on surfaces and in the air.
Using the data in the current section, of a group of 100 people how many would, on average, be found to carry one of Streptococcus pyogenes, Streptococcus pneumoniae or Clostridium difficile? Why are individuals carrying these microbes generally not ill?
Streptococcus pyogenes is found in one-tenth of the population, so that means that 10 people in every hundred would carry it. Streptococcus pneumoniae occurs in one-fifth of the population, which is 20 in every hundred individuals and Clostridium difficile is found in 3% of the population, 3 in every hundred people. So if each person only carried one species, then 33 people in every hundred (33%) of the population would be infected with one of the three species. In fact, probably some people will carry two, and a few might be harbouring all three, so the number of infected people is likely to be less than 33 in every hundred. As long as the people carrying these microbes have immune systems which are functioning efficiently, they are not taking antibiotics and they do not have any wounds or similar injuries, the microbes are kept under control and do not multiply and cause disease. However, when patients are admitted to hospital they are increasingly screened (tested) for these microbes so that precautions can be taken to prevent the spread of infection.
Equally infamous is Escherichia coli, a Gram-negative bacterium found naturally living in the gut. E. coli normally colonises the gut within the first two days after birth. Like so many bacteria, usually it is a harmless commensal, but there are some strains of the bacterium that are harmful. One of the most well known is E. coli strain O157, which can cause serious food poisoning by producing toxins. In young children and the elderly or in people whose immune system is not functioning properly, it can be fatal.
Copyright & revisions
Originally published: Wednesday, 2nd March 2016
Last updated on: Wednesday, 2nd March 2016
- Creative-Commons: The Open University is proud to release this free course under a Creative Commons licence. However, any third-party materials featured within it are used with permission and are not ours to give away. These materials are not subject to the Creative Commons licence. See terms and conditions. Full details can be found in the Acknowledgements and our FAQs section.
- This site has Copy Reuse Tracking enabled - see our FAQs for more information.
If you enjoyed this, why not follow a feed to find out when we have new things like it? Choose an RSS feed from the list below. (Don't know what to do with RSS feeds?)
Remember, you can also make your own, personal feed by combining tags from around OpenLearn.
All our alternative formats are free for you to download, for more information about the different formats we offer please see our FAQs. The most frequently used are Word (for accessibility), PDF (for print) and ePub and Kindle to download to eReaders*.
- Word (1.4 MB)
- PDF (3.1 MB)
- ePub 3.0 (1.2 MB)
- ePub 2.0 (1.2 MB)
- Kindle (457 KB)
- RSS (99 KB)
- HTML (892 KB)
- SCORM (892 KB)
- OUXML Package (17 KB)
- OUXML File (46 KB)
- IMS Common cartridge
- Moodle backup (892 KB)
*Please note you will need an ePub and Mobi reader for these formats.