Bacteria known as intestinal flora survive in both the small and large intestine of the human body—thanks to the neutral pH of the intestinal environment. Because these bacteria have low requirements for oxygen and sunlight exposure, they are well suited to life in this environment. They use our digested food as a source of nutrition and even provide a benefit to us by synthesizing several of the vitamins we need, such as biotin, as well as vitamins K and B-5. In addition, if these bacteria are thriving, it is more difficult for disease-causing microbes to establish themselves in the intestines and attack our bodies.