A DAZZLING menagerie of microbes live inside the human gut—by some counts a few thousand different species. Most residents of this gut microbiome are not the disease-causing kind. In fact, many do useful jobs, such as breaking down certain carbohydrates, fibres and proteins that the human body would otherwise struggle to digest. Some even produce essential compounds the body cannot make on its own, like B vitamins and short-chain fatty acids, which help regulate inflammation, influence the immune system and affect metabolism.