Factors affecting metabolic activity of the intestinal microflora.

1. The metabolic activity of the gastrointestinal microflora can be modified by numerous factors derived from the host, the host's environment, and the flora itself. 2. Marked differences exist in microbial composition and metabolism of the gut floras of different species of animal, and in the degree of colonization of the various regions of the gut. 3. The microbial flora changes with age, particularly in early infancy, and developmental changes in in vivo metabolism attributable to the gut flora have been noted for methylmercury. 4. Gastrointestinal disease can have a profound effect on microbial colonization and metabolism in the upper regions of the human gut. In particular, hypochlorhydria can lead to elevation in bacterial numbers, and nitrite and nitrosamine concentrations in gastric juice. 5. Environmental factors such as drugs (particularly antibiotics), diet, and xenobiotics can modify microbial metabolism and toxicity of foreign compounds. In some cases exposure to xenobiotics (e.g., cyclamate, rutin) results in metabolic adaptation and increased biotransformation of the foreign compound. 6. By their fermentation of dietary residues and endogenous carbohydrates, the gut bacteria produce acids which can lower lumenal pH and, in turn, modify bacterial enzyme activities. However, the changes in activity appear to be enzyme specific, some increasing with pH and others being inversely related to pH of incubation.