Where are the bacteria on our bodies?

It is true, we are passengers in our own body. In fact, 10 is a conservative estimate. There are 50X more bacterial cells living in you and on you than there are cells in the rest of your body that are you.

Most of them are in the gut, of course, the intestines. We have them there because we have the perfect place for them to live. There's lots of things for them to eat; there's lots of gases which they can metabolise. Most of our insides are devoid of oxygen, for example, and lots of these bugs are anaerobes (they don't like oxygen). They do us a lot of favours and by having them there they're taking up resources and space which potentially nasty bacteria could take up. By having lots of these bugs in us they're in fact protecting us from being infected. When you go abroad and get diarrhoea what's actually happening is that the local bugs are combating your friendly bugs and beating them off for a while. Your body then learns to react and pushes the nasty ones back out so the friendly ones can re-colonise. Bacteria are roughly a tenth of the size, or smaller, than a human cell. In fact, if you look inside a human cell you can see evidence of evolution because there are these structures called mitochondria. Mitochondria are the cellular powerhouses. The mitochondria are the same size as a bacterium and scientists think they look so similar because way back in evolution a bacterium got inside an early cell. The two developed this partnership called the Symbiosis Theory and the bacterium lots of energy but the cell gave the bacterium protection and things it needed. The consequence was the two things lived happily side by side. But we're still living side-by-side with bacteria. We need them and if we don't have them in our guts then we're less healthy for the simple reason that if you rear animals and they're not allowed to have bacteria in their guts they don't do very well. This can mean that taking antibiotics may have negative effects. When you take antibiotics that are 'broad-spectrum' antibiotics these go through you like Domestos and kill the good bacteria. Anything that's left behind that's not vulnerable to the effect of antibiotic can then over grow. Yeasts and other fungi infections can do that because antibiotics won't kill those but they will kill the bacteria. Things like Clostridium difficiles, C. diff which leads to antibiotic associated diarrhoea and kills people in hospital. If people are but on heavy dose antibiotics then all the friendly bacteria get wiped out and these other ones over grow. It can be a big problem.