In humans, the gut microbiota has the largest numbers of bacteria and the greatest number of species compared to other areas of the body. The composition of human gut microbiota changes over time, when the diet changes, and as normal role of these enzymes in bacterial cells health changes. The microbial composition of the gut microbiota varies across the digestive tract. Many species in the gut have not been studied outside of their hosts because most cannot be cultured. While there are a small number of core species of microbes shared by most individuals, populations of microbes can vary widely among different individuals.