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Good and bad bacteria

Why do some bacteria cause us harm while others are good for us?
A labelled diagram of a typical bacterium cell
© Wellcome Genome Campus Advanced Courses and Scientific Conferences
In the media, we often hear about good and bad bacteria. What do we mean by this?
We cannot escape bacteria, they are everywhere. While a few species cause disease most have no effect on us and many are very useful to us. Our bodies are home to trillions of bacterial cells and it is thought that we contain more bacterial cells than human cells. Many of these probably do nothing detrimental, but nor do they do us any good. These are known as commensal bacteria. Others, particularly those in our gut, obtain nutrients from us, but also help us to break down food and protect us from ‘bad bacteria’. These are known as symbiotic, or ‘good bacteria’.
Bad bacteria are those which do us harm. We refer to them as pathogens. These might be acquired from our environment. The bacterium causing cholera, Vibrio cholerae, can be acquired by eating food or drinking water contaminated with the faeces of infected people or through direct contact with an infected person. Streptococcus, which can cause sore throats, is caught through contact with infected saliva, such as through coughing. Some bacteria only become pathogenic when they get the chance. These are called opportunistic pathogens. Staphylococcus aureus bacteria (such as MRSA) commonly live all over our bodies with no ill effect. But when given the chance, such as when the skin is broken during surgery, they can infect the skin and other tissues.
We cannot avoid the fact that bacteria are essential for our good health, but that they can also cause devastating disease. The trick is to discover interventions and treatments that target the bad bacteria and leave us living happily with the good ones.
© Wellcome Genome Campus Advanced Courses and Scientific Conferences
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Bacterial Genomes: Disease Outbreaks and Antimicrobial Resistance

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