<Table> <Tr> <Td> Tryptophan metabolism by human gastrointestinal microbiota (v t e) This diagram shows the biosynthesis of bioactive compounds (indole and certain derivatives) from tryptophan by bacteria in the gut . Indole is produced from tryptophan by bacteria that express tryptophanase . Clostridium sporogenes metabolizes indole into 3 - indolepropionic acid (IPA), a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals . In the intestine, IPA binds to pregnane X receptors (PXR) in intestinal cells, thereby facilitating mucosal homeostasis and barrier function . Following absorption from the intestine and distribution to the brain, IPA confers a neuroprotective effect against cerebral ischemia and Alzheimer's disease . Lactobacillus species metabolize tryptophan into indole - 3 - aldehyde (I3A) which acts on the aryl hydrocarbon receptor (AhR) in intestinal immune cells, in turn increasing interleukin - 22 (IL - 22) production . Indole itself acts as a glucagon - like peptide - 1 (GLP - 1) secretagogue in intestinal L cells and as a ligand for AhR . Indole can also be metabolized by the liver into indoxyl sulfate, a compound that is toxic in high concentrations and associated with vascular disease and renal dysfunction . AST - 120 (activated charcoal), an intestinal sorbent that is taken by mouth, adsorbs indole, in turn decreasing the concentration of indoxyl sulfate in blood plasma . </Td> </Tr> </Table> <Tr> <Td> Tryptophan metabolism by human gastrointestinal microbiota (v t e) This diagram shows the biosynthesis of bioactive compounds (indole and certain derivatives) from tryptophan by bacteria in the gut . Indole is produced from tryptophan by bacteria that express tryptophanase . Clostridium sporogenes metabolizes indole into 3 - indolepropionic acid (IPA), a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals . In the intestine, IPA binds to pregnane X receptors (PXR) in intestinal cells, thereby facilitating mucosal homeostasis and barrier function . Following absorption from the intestine and distribution to the brain, IPA confers a neuroprotective effect against cerebral ischemia and Alzheimer's disease . Lactobacillus species metabolize tryptophan into indole - 3 - aldehyde (I3A) which acts on the aryl hydrocarbon receptor (AhR) in intestinal immune cells, in turn increasing interleukin - 22 (IL - 22) production . Indole itself acts as a glucagon - like peptide - 1 (GLP - 1) secretagogue in intestinal L cells and as a ligand for AhR . Indole can also be metabolized by the liver into indoxyl sulfate, a compound that is toxic in high concentrations and associated with vascular disease and renal dysfunction . AST - 120 (activated charcoal), an intestinal sorbent that is taken by mouth, adsorbs indole, in turn decreasing the concentration of indoxyl sulfate in blood plasma . </Td> </Tr> <P> The gut flora 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 fermentating 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>

Where is the most bacteria found in the human body