Many species of bacteria have evolved and adapted to live and grow in the human intestine. The intestinal habitat of
an individual contains 300–500 different species of bacteria,1,2 and the number of microbial cells within the gut lumen is about 10 times larger than the number of eukaryotic cells in the human body.3 The stomach and small intestine contain only a few species of bacteria adhering to the epithelia and some other bacteria in transit. The scarcity of bacteria in the upper tract seems to be because of the composition of the luminal medium (acid, bile, pancreatic secretion), which kills most ingested microorganisms, and because of the phasic propulsive motor activity towards the ileal end, which impedes stable colonisation of bacteria in the lumen. By contrast, the large intestine contains a complex and dynamic microbial
ecosystem with high densities of living bacteria, which achieve concentrations of up to 1011 or 1012 cells/g of luminal contents.1 These concentrations are similar to those found in colonies growing under optimum conditions over the surface of a laboratory plate.4 A large proportion of the faecal mass consists of bacteria (around 60% of faecal solids)