Increased mutability by oxidative stress in OxyR-deficient Escherichia coli and Salmonella typhimurium cells: clonal occurrence of the mutants during growth on nonselective media.

Escherichia coli and Salmonella typhimurium strains deficient in the OxyR-regulated adaptive response to oxidative stress were used to study the mode in which spontaneous SOS-dependent mutations are generated in a distressed bacterial population. When assayed on supplemented selective medium, the E. coli strain IC3821 (trpE65), carrying the delta oxyR30 mutation and containing the plasmid pRW144 (mucA/B), showed a frequency of spontaneous Trp+ revertants similar to that of the oxyR+ control. Instead, the IC3821 strain exhibited an enhancement in the clonal occurrence of spontaneous revertants arising at random during growth on a nonselective medium. A similar enhancement was observed for the S. typhimurium strain TA4125 (hisG428 delta oxyR2). The mutator effect observed in oxyR- cells would be induced by an increased background of reactive oxygen species; it provides a model for studying the mutability of a cell population constantly exposed to mutation-inducing agents. In the IC3821 strain, revertants were induced by t-butyl hydroperoxide with higher efficiency than in oxyR+. We suggest that strain IC3821 could be useful for the detection of SOS-dependent mutagenesis induced by chemical oxidants.