The dominant paradigm for the evolution of mutator alleles in bacterial populations is that they spread by indirect selection for linked beneficial mutations when bacteria are poorly adapted. the first experimental evidence that direct selection can favour mutator alleles in bacterial populations, and pave the way for BEZ235 manufacturer future studies to understand how mutation and DNA repair are linked to stress responses and how this affects the evolution of bacterial mutation rates. mutant displays altered expression of a small number of housekeeping genes [16], raising the possibility that direct fitness costs and benefits may be associated with mutator alleles as a result of the pleiotropic effects of mutator alleles on gene expression. While the initial goal of this study was to investigate the interplay between BEZ235 manufacturer phenotypic and genetic changes in mutation rates in response to stress, preliminary findings led us to study the impact of direct stress-imposed selection on mutator evolution. We used a model system involving oxidative stress in wild-type and mutator strains of mutator. Exposure to oxidative stress causes an increase in mutation rate in the wild-type strain, but mutators do not display stress-induced increases in mutation rate as a result of their intrinsic resistance to hydrogen peroxide. Direct competition experiments between mutators and wild-type confirm the nature of public good of the protective enzymes, as both strains share the benefits when exposed to the stress. 2.?Results (a) Mutation rate and stress resistance To test for direct selection on mutator alleles, we measured the mutation rate and hydrogen peroxide resistance of a wild-type strain of and isogenic mutator strains carrying deletions in DNA repair pathways that are commonly isolated in natural populations (= 0.9777, d.f. = 3, = 0.0222). Because we measured hydrogen peroxide resistance over a short time period (15 min of exposure) and the strains Eno2 were constructed for this study, we can be confident that this increased resistance of mutators reflects intrinsic resistance of mutator strains, and not an increase in the rate of evolution of resistance in mutator strain. These results demonstrate the presence of a direct benefit associated with mutator alleles in the presence of oxidative stress as a result of a trade-off between DNA repair efficiency and hydrogen peroxide resistance. Open in a separate window Physique?1. Mutation rate, hydrogen peroxide resistance and catalase activity of mutators and wild-type (WT). (cultures prior BEZ235 manufacturer to exposure to hydrogen peroxide as measured using the Amplex red reagent-based assay. Significant differences are represented by asterisks (* 0.05, ** 0.01 and *** 0.001, respectively) when suitable and not significant (n.s.) when not. Plotted data show the mean of (= 0.1936), except for = 0.0143). This suggests that an elevated catalase secretion is the BEZ235 manufacturer biochemical mechanism underlying the hydrogen peroxide resistance in the case of and strains have elevated hydrogen peroxide mechanisms as a result of the upregulation of alternative pathways for hydrogen peroxide resistance found in [19]; alternatively, it is possible that these mutators have elevated catalase secretion, but that our assay lacked the power to detect the difference in catalase secretion between these poor mutators and the wild-type. (b) Stress and mutation rate To measure the impact of stress on the mutation rate of wild-type and mutators, we measured the mutation rate after exposure to hydrogen peroxide (physique 2 0.05). Mutators, on the other hand, showed a one-third-fold decrease in mutation rate in the case of the strong mutator ( 0.001) and no significant change for the mild and weak mutators. The consequence of this is that this increase in mutation rate associated with strong mutator alleles is dependent on stress. Open in a separate window Physique?2. Mutation rate and hydrogen peroxide effect on cell density in mutators and wild-type (WT). (strains (black bars) and effect of hydrogen peroxide treatment on mutation rate in non washed and washed cells (light and dark grey bars). Significant differences between untreated and both treatment are represented (n.s., * 0.05, ** 0.05, *** 0.01 and 0.001, respectively). Error bars are confidence intervals as calculated using the Ma-Sandri-Sarkar maximum-likelihood method. In (and and but decreased cell density in and wild-type around 40 per cent and 60 per cent, respectively (physique 2mutator.