The "normal" flora of the intestine represents an important defense against pathogens which colonize the gut. This flora can also be harmful, e.g., by constantly invading the body, or by producing carcinogens. Numerous workers have tried to control this flora, such that beneficial species are increased, and harmful ones eliminated. Such earlier studies were unsuccessful because (1) interactions among bacteria are dependent on the environment in which they are studied, and (2) numerous individual mechanisms simultaneously control the populations of intestinal bacteria. Dr. Freter has overcome some of these difficulties by (1) using model systems (anaerobic continuos flow cultures, germfree mice) which do simulate the normal intestinal environment, and (2) making mathematical models of host bacterium interactions which can evaluate the simultaneous effects of ALL known mechanisms that control indigenous bacteria.