<P> Gut flora, or 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 . </P> <P> 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 . </P> <P> The relationship between some gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship . 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 . Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols, and xenobiotics . The systemic importance of the SCFAs and other compounds they produce are like hormones and the gut flora itself appears to function like an endocrine organ, and dysregulation of the gut flora has been correlated with a host of inflammatory and autoimmune conditions . </P> <P> The composition of human gut microbiota changes over time, when the diet changes, and as overall health changes . A systematic review from 2016 examined the preclinical and small human trials that have been conducted with certain commercially available strains of probiotic bacteria and identified those that had the most potential to be useful for certain central nervous system disorders . </P>

What vitamin is produced by bacteria in the gut