Mutations in Salmonella typhimurium recovered from livers and spleens of mice

Balb/c mice were inoculated intraperitoneally with TA2662, a smooth derivative of the Salmonella typhimurium Ames tester strain (TA102) which carries the mutable hisG locus on a multicopy plasmid, or TA103, which carries the same hisG gene on the chromosome. The bacteria were recovered at various times from the livers and spleens of the infected mice. Total numbers of bacteria were determined and the mutant frequency was estimated. The frequency of occurrence of histidine prototrophs in experiments using TA2662 was substantially above the frequency found with this strain grown in vitro. The mutant frequencies in experiments using TA103 recovered from mice were also highly significantly increased above background. We did not identify factors which might suggest selection in vivo for histidine prototrophs. There is sufficient histidine in body fluids of the host for the growth of His- bacteria. The His- and His+ derivatives were found to grow equally well in vitro in the presence of amounts of histidine approximating concentrations known to exist in vivo. It is probable that mutations in TA2662 are greatly underestimated, since the hisG-containing plasmid is lost at relatively high frequency during incubation in a variety of conditions.