After the absorption of nutrients in the small intestine, water and inorganic ions are absorbed in the large intestine, or colon, of the hindgut (see Fig. 40.12). Peristalsis moves the contents of the small intestine into the colon at a pace that allows time for nutrient absorption in the small intestine. By the time the gut contents reach the large intestine, the nutrients have been absorbed into the body, but water and inorganic ions remain. These are absorbed in the large intestine until the contents form semisolid feces. Excess water absorption can cause constipation, whereas too little water absorption results in diarrhea. Diarrhea and constipation also can be caused by toxins released by microorganisms, such as the bacterium Vibrio cholera, the cause of cholera, in the food an animal eats. Feces are stored in the final segment of the colon, the rectum, until they are periodically eliminated through the anus.

As we saw in Case 5: The Human Microbiome, digestion is not a solo endeavor. Large populations of bacteria reside in the small and large intestines and help extract nutrients that the animal’s body cannot extract itself. The principle gut resident is Escherichia coli, the bacterium commonly used for research in molecular biology laboratories. In the process of nourishing themselves by aiding in the digestion of the host’s gut contents, the bacteria provide nutrients and certain vitamins, such as biotin and vitamin K, that the animal cannot produce itself. Vitamin deficiency therefore can sometimes result from prolonged antibiotic medication that kills large numbers of gut bacteria. Another by-product of bacterial action in the gut is gases such as methane and hydrogen sulfide. These are expelled when foods that, like beans, contain compounds that the bacteria can digest but that the host animal cannot. The mutual benefits to the host animal and the bacteria ensure the success of their symbiotic relationship.