Fig. 2.

Microbial diversity and composition across gut sections (adapted and modified from [29]). The gut microbiota varies with pH and oxygen tension, flow rates of digestive fluids, substrate availability, and host secretions. pH plays a key role in microbial community dynamics. The stomach and proximal small intestine are particularly unfavorable for bacterial residence, and very few bacteria are resistant to the acidic microenvironment. The highest biodiversity is in the colon, in which the pH is around 5.