Question: how does bacteria adapt to our human bodies?

Laura Nolan answered on 1 Feb 2019:

We have many bacteria that live in our guts, in our mouth and on our skin. These bacteria are in those places basically all of our lives, and don’t cause us any harm (these bacteria are called commensals). Instead they help us by stopping bacteria that could harm us from infecting these parts of our body. The commensal bacteria that live in these places are able to live in that location because they have adapted to the particular conditions there eg. low pH and low oxygen in the guts, high fluid movement in the mouth and dry, salty conditions on the skin. They also need to be able to use the nutrients in that environment as a food source to live and grow. Bacteria adapt to conditions by changing either which genes (parts of DNA) are producing proteins, or how much protein they are making, and where it is within their cells. This can help them sense what the environment is like around them, then they can make whatever proteins are needed to survive those conditions. If bacteria can’t survive in the condition (ie they have no way to adapt to this change) they will either die, or a small part of the population will find a way to survive by getting mutations in their DNA, that allow them to change something about their cells to mean they can tolerate the environmental conditions and thus be adapted to that environment.