The relationship between gut flora and humans is a mutualistic, symbiotic relationship. These microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful species, regulating the development of the gut, producing vitamins for the host such as biotin and vitamin K, and producing hormones to direct the host to store fats. However, in certain conditions, some species are thought to be capable of causing disease or increasing cancer risk for the host.
Although the acid in the stomach, as well as bile and pancreatic secretions, hinder colonization of most bacteria in the stomach and proximal small intestine, most of the gut flora are found in the distal small intestine and in the cecum and ascending colon. In the small intestine there are Gram-positive cocci (bacteria), while those in the colon are mostly Gram-negative. The first part of the colon is mostly responsible for fermenting carbohydrates, while the latter part mostly breaks down proteins and amino acids. Bacterial growth is rapid.