Resistance of the Mouse's Intestinal Tract to Experimental Salmonella Infection I. Factors Which Interfere with the Initiation of Infection by Oral Inoculation* by Marjorie Bohnhoff, C. Phillip

Described below are observations which seem to account, in large part at least, for the normal resistance of the mouse's intestinal tract to infection with Salmonella enteritidis introduced by mouth, the natural route of infection. Thus inoculated, about 106 microorganisms are required to infect 50 per cent of young adult CF-1 mice (1). Their resistance, however, can be sharply reduced by the oral administration of a single, large dose of streptomycin, for during the following 24 hours, < 10 microorganisms of the same strain suffice to initiate infection (2, 3). Of the changes in the mouse's enteric microflora resulting from streptomycin treatment, the most consequential was thought to be the elimination of certain obligate anaerobes belonging to the genus Bacteroides (4, 5). The experiments to be described demonstrate that multiplication of S. enteritidis was inhibited in vitro by: (a) centrifuged supernatants of heatkilled suspensions of colon content or feces of normal, i.e. untreated mice; (b) anaerobic cultures of such materials; (c) anaerobic cultures of Bacteroides isolated from them; (d) two fatty acids (acetic and butyric) recovered from bowel content and also from cultures of Bacteroides. In all these experiments, the degree of inhibitory activity was always dependent upon pH and was always greater under anaerobic conditions which simulated those within the colon content in vivo. As will be shown in the following communication inhibitory activity was never demonstrable with suspensions of colon content or feces of mice treated by mouth with streptomycin on the preceding day. The following paper (6) also describes changes in the chemical constituents and the pH of colon content which follow such treatment and are associated with enhanced susceptibility.