Gut microbiota / gut microbiome

The gut microbiota comprises approximately 38 trillion bacteria — plus archaea, fungi, viruses and other microorganisms — that colonise the human gastrointestinal tract, with the highest density in the colon. Collectively they encode a gene catalogue roughly 150-fold larger than the human genome and perform functions the host cannot accomplish alone, including fermentation of dietary fibre into short-chain fatty acids, synthesis of certain B vitamins and vitamin K2, modulation of bile acid chemistry, and calibration of mucosal immunity. Compositional and functional differences between individuals are large — shaped by birth mode, infant feeding, diet, geography, antibiotics and age — and these inter-individual differences complicate the search for universal 'optimal' compositions. The term 'microbiome' technically encompasses both the organisms and their collective genetic material, but the two words are widely used interchangeably in the clinical literature.