Click the link for a quite long, quite interesting, article about how humans are incredibly reliant on the bacteria that we're full of. For a start:
A healthy adult human harbours some 100 trillion bacteria in his gut alone. That is ten times as many bacterial cells as he has cells descended from the sperm and egg of his parents. These bugs, moreover, are diverse. Egg and sperm provide about 23,000 different genes. The microbiome, as the body’s commensal bacteria are collectively known, is reckoned to have around 3m. Admittedly, many of those millions are variations on common themes, but equally many are not, and even the number of those that are adds something to the body’s genetic mix.The article then gives many examples of ways in which the bacteria may affect our health. The one I found most surprising:
The link with diabetes was noticed in morbidly obese people who had opted for a procedure known as Roux-en-Y, which short-circuits the small intestine and thus reduces the amount of food the body can absorb. Such people are almost always diabetic. As a treatment for obesity, Roux-en-Y is effective. As a treatment for diabetes, it is extraordinary. In 80% of cases the condition vanishes within days.
Experiments conducted on mice by Dr Nicholson and his colleagues show that Roux-en-Y causes the composition of the gut microbiome to change. Dr Nicholson thinks this explains the sudden disappearance of diabetes.
The diabetes in question is known as type-2. It is caused by the insensitivity of body cells to insulin, a hormone that regulates the level of blood sugar. Insulin sensitivity is part of a complex and imperfectly understood web of molecular signals. Dr Nicholson suspects, though he cannot yet prove, that some crucial part of this web is regulated by the microbiome in a way similar to the role played by formic acid in the case of high blood pressure. The intestinal bypass, by disrupting the microbiome, resets the signal, and the diabetes vanishes.