Gut flora (gut microbiota, or gastrointestinal microbiota) is the complex community of microorganisms that live in the digestive tracts of humans and other animals, including insects. The gut metagenome is the aggregate of all the genomes of gut microbiota. The gut is one niche that human microbiota inhabit.
In humans, the gut microbiota has the largest numbers of bacteria and the greatest number of species compared to other areas of the body. In humans the gut flora is established at one to two years after birth, and by that time the intestinal epithelium and the intestinal mucosal barrier that it secretes have co-developed in a way that is tolerant to, and even supportive of, the gut flora and that also provides a barrier to pathogenic organisms
The relationship between some gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship.:700 Some human gut microorganisms benefit the host by fermenting dietary fiber into short-chain fatty acids (SCFAs), such as acetic acid and butyric acid, which are then absorbed by the host