PENICILLIN-SCREENED AUXOTROPHIC MUTATIONS IN SALMONELLA TYPHIMURIUM AND THEIR RELATION TO X-RAY DOSAGE

Abstract

Suspensions of S. typhimurium have been exposed to successive dosages of X-radiation up to 57,000 r units, then screened with penicillin in minimal medium (salts and glucose) to kill off most of the unaffected cells which multiply in it. Centrifuged organisms were resuspended and plated on complete medium containing amino acids, vitamins, and nucleic acid; and colonies gave strains which require one or another specific nutrilite that the parent strain could synthesize for itself. The frequencies of these auxotrophic mutations are correlated with the X-ray dosage, indicating that they are gene mutations similar to those produced in higher organisms. A test of the efficacy of penicillin screening on artificially made mixtures of wild and an auxotrophic mutant confirms the assumption that wild type strains survive a 24-hr. penicillin exposure in proportion to their initial frequency. Among the mutations isolated,the most common following the lower X-ray dosages was a cysteine requiring auxotroph, whereas after higher dosages a histidine requiring auxotroph predominated. Preliminary tests for recombination of mutants in mixtures indicate that this phenomenon does not occur in Salmonella.