Effects of antimicrobial agents upon the functional part of the intestinal flora.

For functional studies of the complex intestinal ecosystem, two recently introduced terms have been shown to be of value. A Microflora-Associated Characteristic--MAC--is defined as the recording of any anatomical structure, physiological or biochemical function in a macroorganism which has been influenced by the microflora. When functionally active microbes are absent, as in germfree animals, the recording of a MAC can be defined as a Germfree Animal Characteristic--GAC. In our ongoing, long-term ecological studies we have investigated the influence of a long list of antimicrobial agents upon various biochemical MACs in man and rats. Parameters such as conversion of cholesterol to coprostanol, 7-dehydroxylation of bile acids, transformation of bilirubin, degradation of mucin, production of short chain fatty acids and inactivation of tryptic activity have been the MACs most often studied. Each antibiotic may create its own MAC/GAC profile, partly reflecting its antibacterial spectrum and concentration in the intestinal tract and partly the characteristics of the microbial species actually involved in the reaction. MACs influenced mainly by species dominated by a K-selection are generally more disturbed than MACs influenced by species dominated by a r-selection.