<P> A small number of bacteria and fungi are normally present in the conjunctiva . Classes of bacteria include Gram - positive cocci (e.g., Staphylococcus and Streptococcus) and Gram - negative rods and cocci (e.g., Haemophilus and Neisseria) are present . Fungal genera include Candida, Aspergillus, and Penicillium . The lachrymal glands continuously secrete, keeping the conjunctiva moist, while intermittent blinking lubricates the conjunctiva and washes away foreign material . Tears contain bactericides such as lysozyme, so that microorganisms have difficulty in surviving the lysozyme and settling on the epithelial surfaces . </P> <Table> <Tr> <Td> Tryptophan metabolism by human gastrointestinal microbiota (<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul>) Tryptophan Clostridium sporogenes Lacto - bacilli Tryptophanase - expressing bacteria IPA I3A Indole Liver Brain IPA I3A Indole Indoxyl sulfate AST - 120 AhR Intestinal immune cells Intestinal epithelium PXR Mucosal homeostasis: ↓ TNF - α ↑ Junction protein - coding mRNAs L cell GLP - 1 TJ Neuroprotectant: ↓ Activation of glial cells and astrocytes ↓ 4 - Hydroxy - 2 - nonenal levels ↓ DNA damage--Antioxidant--Inhibits β - amyloid fibril formation Maintains mucosal reactivity: ↑ IL - 22 production Associated with vascular disease: ↑ Oxidative stress ↑ Smooth muscle cell proliferation ↑ Aortic wall thickness and calcification Associated with chronic kidney disease: ↑ Renal dysfunction--Uremic toxin Kidneys This diagram shows the biosynthesis of bioactive compounds (indole and certain other derivatives) from tryptophan by bacteria in the gut . Indole is produced from tryptophan by bacteria that express tryptophanase . Clostridium sporogenes metabolizes tryptophan into indole and subsequently 3 - indolepropionic acid (IPA), a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals . IPA binds to the pregnane X receptor (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 triggers the secretion of glucagon - like peptide - 1 (GLP - 1) in intestinal L cells and acts 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 (<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul>) Tryptophan Clostridium sporogenes Lacto - bacilli Tryptophanase - expressing bacteria IPA I3A Indole Liver Brain IPA I3A Indole Indoxyl sulfate AST - 120 AhR Intestinal immune cells Intestinal epithelium PXR Mucosal homeostasis: ↓ TNF - α ↑ Junction protein - coding mRNAs L cell GLP - 1 TJ Neuroprotectant: ↓ Activation of glial cells and astrocytes ↓ 4 - Hydroxy - 2 - nonenal levels ↓ DNA damage--Antioxidant--Inhibits β - amyloid fibril formation Maintains mucosal reactivity: ↑ IL - 22 production Associated with vascular disease: ↑ Oxidative stress ↑ Smooth muscle cell proliferation ↑ Aortic wall thickness and calcification Associated with chronic kidney disease: ↑ Renal dysfunction--Uremic toxin Kidneys This diagram shows the biosynthesis of bioactive compounds (indole and certain other derivatives) from tryptophan by bacteria in the gut . Indole is produced from tryptophan by bacteria that express tryptophanase . Clostridium sporogenes metabolizes tryptophan into indole and subsequently 3 - indolepropionic acid (IPA), a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals . IPA binds to the pregnane X receptor (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 triggers the secretion of glucagon - like peptide - 1 (GLP - 1) in intestinal L cells and acts 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> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul>

Where are most of the normal microbiota of the digestive system found