http://en.wikipedia.org/wiki/Gut_flora "Though people can survive without gut flora,[4] the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful, pathogenic bacteria,[2] regulating the development of the gut, producing vitamins for the host[citation needed] (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 by producing infection or increasing cancer risk for the host."

http://futureboy.us/frinkdocs/