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Microbes – friend or foe?
Microbes – friend or foe?

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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.

Question 1

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.