Microflora of the Gastrointestinal Tract: A Review

The mucosal surface of the human gastrointestinal (GI) tract is about 200–300 m² and is colonized by 10^13–14 bacteria of 400 different species and subspecies. Savage (1 ) has defined and categorized the gastrointestinal microflora into two types, autochthonous flora (indigenous flora) and allochthonous flora (transient flora). Autochthonous microorganisms colonize particular habitats, i.e., physical spaces in the GI tract, whereas allochthonous microorganisms cannot colonize particular habitats except under abnormal conditions. Most pathogens are allochthonous microorganisms; nevertheless, some pathogens can be autochthonous to the ecosystem and normally live in harmony with the host, except when the system is disturbed (2 ).