A dazzling menageries microbes live inside the hunman gut——by some counts a few  thousand different species. Most residents o 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 disgest. Some even product 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.

纷繁多样的微生物群落居于人类肠道中——据某些统计，种类多达数千。这个肠道微生物群中的大多数居民并非治病种类。事实上，许多微生物做着有益的工作，例如分解人体某些自身难以消化的碳水化合物、纤维和蛋白质。有些微生物甚至能产生人体自身无法制造的重要化合物，如B族维生素和短链脂肪酸，这些物质有助于调节炎症、影响免疫系统并作用于新陈代谢。