Interactions among microorganisms of the indigenous intestinal flora and their influence on the host.

The animal host and its intestinal microbial flora function together as a complex ecologic system in which there is a significant impact of the intestinal flora on the host as well as of components of the microbial flora on one another. Aerobic and anaerobic bacteria of the intestinal flora influence numerous anatomic, physiologic, and immunologic parameters of the host. Constituents of the indigenous intestinal flora also engage in a multitude of antagonistic and cooperative interactions. The normal bacterial intestinal flora represents an extremely important defense mechanism, which effectively interferes with the establishment of many important enteric pathogens. Mechanisms by which microorganisms suppress the growth of other microorganisms include modification of bile acids, stimulation of peristalsis, induction of immunologic responses, depletion of essential substrates from the environment, competition for attachment sites, creation of restrictive physiologic environments, and elaboration of antibiotic-like substances. Components of the intestinal microbial flora also interact synergistically in the induction of disease or the utilization of substrate.