Human gut flora

The human gut flora, also referred to as the gut microbiota or intestinal microbiome, comprises the vast and diverse community of microorganisms — predominantly bacteria, but also archaea, fungi, viruses, and protists — that colonize the gastrointestinal tract. Estimated to harbor trillions of microbial cells encoding tens of millions of genes, this ecosystem represents one of the most complex and metabolically active environments in the human body. The dominant bacterial phyla in healthy adults include Firmicutes, Bacteroidota (formerly Bacteroidetes), Actinobacteria, and Proteobacteria, with their relative abundances shaped by host genetics, age, diet, geographic origin, and pharmaceutical exposures.

Far from being passive residents, gut microorganisms perform critical physiological functions: fermenting dietary fiber into short-chain fatty acids (SCFAs), synthesizing vitamins, educating and calibrating the host immune system, maintaining intestinal barrier integrity, modulating bile acid metabolism, and producing a broad repertoire of bioactive metabolites that circulate systemically. Disruption of this community — termed dysbiosis — has been linked to an expanding spectrum of conditions extending well beyond the gastrointestinal tract, including metabolic disorders, neurodegenerative diseases, cancers, autoimmune conditions, and psychiatric illnesses. The gut microbiota's capacity to influence remote organ systems via the gut-liver axis, gut-lung axis, and microbiota-gut-brain axis has positioned it as a central subject in translational medicine and multi-omics research.