Properties of the Ames Salmonella mutants lodged in the gastrointestinal tract of gnotobiotic rats.

An association of the histidine auxotroph of Salmonella typhimurium (strain TA1538) within the gastrointestinal tract of otherwise germ-free Sprague-Dawley rats is maintained during observations for up to 7 months. The bacteria exceed concentrations of 10(7)/g in the forestomach and exceed concentrations of 10(8)/g in the lower bowel and feces. When carcinogens are ingested, the number of revertants in the feces increases. The ingestion of structurally related compounds which are not mutagenic to the bacteria in vitro and for which no evidence of carcinogenicity exists does not increase the number of revertants in the feces. The numbers of salmonella are increased by the addition of Lactobacillus plantarum and Bacteroides fragilis but the salmonella disappear from the gastrointestinal tract when the rats are conventionalized. With the additional flora, there is a decrease in the number of revertants appearing in the feces in response to a given dose of carcinogen. This decrease may reflect an effect of the flora on the activity of the metabolic pathway responsible for the presence of the ultimate carcinogen or it may simply be an effect on the salmonella mutants themselves.