Bacteria in the gut fulfill a host of useful functions for humans, including digestion of unutilized energy substrates,stimulating cell growth, repressing the growth of harmful microorganisms, training the immune system to respond only to pathogens, and defending against some diseases.

Colon is the site of salt and water absorption. Glands secrete large quantities of alkaline mucus that l
ubricates the intestinal contents and neutralizes acids formed by bacteria in the intestine. These bacteria aid in the decomposition of undigested food residue, unabsorbed carbohydrates, amino acids, cell debris, and dead bacteria through the process of segmentation and putrefaction. Short-chain fatty acids formed by bacteria from unabsorbed complex carbohydrates provide an energy source for the cells of the left colon. Maintenance of potassium balance is also performed by the colon, wherein the epithelium absorbs and secretes potassium and bicarbonate.