In half of the normal mice examined, cultures of mesenteric lymph nodes were positive for enteric bacteria. When a non-pathogenic microorganism, Serratia marcescens, was established in the intestinal tract by administering it to mice in their drinking water, it, too, was recovered from the mesenteric lymph nodes of almost half of the normal mice examined. From these findings it was concluded that bacteria in small numbers were able to pass from the lumen of the unirradiated gut as far as the regional lymph glands. Such bacteria, except the pathogen, Salmonella, were rarely found in liver or spleen, never in the blood of the normal mice. After x-irradiation with 700 r, the incidence of positive cultures showed the liver or spleen became infected with enteric microorganisms before the blood stream was invaded. It appears, therefore, that these elements of the reticulo-endothelial system were able for a time to maintain the sterility of the blood after failure of the more immediate defenses against bacterial invasion from the intestinal tract. It is concluded that if any increased migration of bacteria through the intestinal mucosa resulted from the radiation injury, the increase must have occurred very soon after irradiation.

Previous studies have shown that the bacteremias which occur in mice during the 2nd week following a moderate dose (600 or 450 r) of total body x-irradiation are caused by members of their normal enteric flora (1). Of the enteric microorganisms, Pseudomonas a~uginosa was shown by serial blood cultures to be the most rapidly lethal once it had invaded the blood stream (2). These observations pointed to the intestinal tract as the site of origin of postirmdiafion bacteremias. They also suggested that Ps. aeruginosa might be a particularly suitablemicroorganism for measuring changes in susceptibility to infection. These indications were substantiated by the observation that Pseudomonas introduced by stomach tube into the gastrointestinal tract of irradiated mice gave rise within a very few days to a high incidence of fatal bacteremlas if inoculation had been made on the 5th or l l t h day post irradiation (3). In normal animals even larger numbers of the same strain of Pse~o-monas, inoculated by the same route, produced no ill effects. This striking difference between normal and irradiated animals in their response to oral inoculation with P s e u J~ seemed to warrant a study of the fate of this microorganism after its introduction into the gastrointestinal tract. Mice were accordingly inoculated on the 5th day post irradiation (550 r) with approximately l0 T Ps. aeruginosa and killed at daily intervals thereafter to determine the numbers of this microorganism in the small and large intestine. It was found that these bacteria were much more likely to become implanted and to multiply in the intestine, particularly in the small intestine, of the irradiated than of the normal mouse. This observation has led to a series of additional experiments undertaken in an attempt to explain this effect of irradiation.

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