ABSTRACT
The human gut harbors a complex and diverse microbiota. The numbers of microorganisms in
the upper gastrointestinal (GI) tract are kept low by the actions of gastric acid, pancreatic
enzymes, bile, and a propulsive motor pattern. The colonic population of microbes is
estimated to be 1012 organisms/gram with at least 400 possible species. The above figure
was obtained by traditional culture-based methods. Modern molecular methods such as 16S
ribosomal RNA clone libraries that are discussed in Chapter 1 indicate that the number of
species will be even higher. The composition of the intestinal microbiota varies from human
to human. These differences in the composition of the microbiota are affected by
physiological, chemical, and environmental factors. The common intestinal microbiota in
humans includes predominantly members of genera Clostridium, Eubacterium, Bacter-
oides, Atopobium and Bifidobacterium spp. and many others to a lesser extent. There is an
approximation that almost 90% of the cells in our body are microbial, whereas only 10%
are human.
