Atoxinknownasheat-stable toxin of E. coli, orSTa,isamajorcausativeagentoftraveler’sdiarrhea,whichcanbecontractedbyconsumptionofinfectedfoodorwater.Thistoxinbindstoareceptorontheapicalsurfaceofintestinalepithelialcellsknownasguanylyl cyclase C (GC-C).Inturnthisenzymegenerateslargequantitiesofintracellularcyclicguanosinemonophosphate(cGMP)thattriggerincreasedCl− secretionviaactivationofthecysticfibrosistransmembraneconductanceregulator(CFTR)Cl− channel.However,onecouldofcoursequestionwhyhumansexpressareceptorforthistoxininasitethatwouldbeaccessibletoluminalbacteriaandtheirproducts.Indeed,thisledtothehypothesisthatthereisanativeligandforGC-Cthatcouldplayaphysiologicalrole.Researcherssubsequentlypurifiedandidentifiedguanylin,ahormonesynthesizedintheintestine.Togetherwitharelatedmolecule,uroguanylin,secretedbythekidney,guanylinisanimportantregulatorofsaltandwaterhomeostasisinthebody.STahasstructuralsimilaritiestoguanylin,butithasmodificationsthatpermitittopersistintheintestinallumenforprolongedperiods.Thisisanexampleofmolecularmimicry,inwhichabacterialproducthijacksareceptorandassociatedsignalingforitsownpurposes(presumablytopropagatethetoxin-producingbacteriatoadditionalhosts).
The colonic microbiota is also notable for its contribution to formation of intestinal gas. Although large volumes of air may be swallowed in conjunction with meals, the majority of this gas returns up the esophagus via belching. However, during fermentation of unabsorbed dietary components, the microbiota generates large volumes of nitrogen, hydrogen, and carbon dioxide. Approximately 1 L of these nonodorous gases is excreted on a daily basis via the anus in all individuals, even those who do not complain of flatulence. Some individuals may generate appreciable concentrations of methane. Trace amounts of odorous compounds are also present, such as hydrogen sulfide, indole, and skatole.
The final stage in the journey taken by a meal after its ingestion is expulsion of its indigestible residues from the body in the process known as defecation. The feces also contain the remnants of dead bacteria, dead and dying epithelial cells that have been desquamated from the lining of the intestine, biliary metabolites specifically targeted for excretion (e.g., conjugates of xenobiotics [see ]), and a small amount of water. In health, the stool contains few if any useful nutrients. The presence of nutrients in stool, particularly lipid (known as steatorrhea), signifies maldigestion, malabsorption, or both. Fat in the stool is a sensitive marker of small intestinal dysfunction because it is poorly used by the colonic microbiota, but loss of carbohydrate and protein in stool can also be seen if the underlying condition worsens.