Multiplication of Salmonella enteritidis was inhibited in vitro by buffered suspensions of fecal material freshly removed from the large intestine of normal mice. Most effective was material obtained from cecum and transverse colon. Inhibitory activity was not impaired by sterilization by heat or filtration. From such materials were isolated acetic and butyric acids in concentrations which inhibited Salmonella in vitro. The degree of inhibitory activity of suspensions of colon content and of mixtures of the two fatty acids was conditioned by pH and favored by anaerobiosis. Effective inhibition occurred at or slightly below the pH of colon content of most normal mice as determined in situ by direct measurement. Acetic and butyric acids were isolated from anaerobic cultures of several strains of Bacteroides previously demonstrated to be one of the most numerous inhabitants of the large intestine of the normal mouse.

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