Resistance to physico-chemical insults in P . putida KT2440 and its Δ ppk and Δ ppx mutant derivatives. (A) Solvent tolerance and heat-shock response. Cells were cultured on LB medium and exposed to the different physicochemical stress agents as detailed in the Methods section. The percentage of survival was calculated as the fraction of colony-forming units after the treatment normalized to those in the untreated control experiment. Error bars represent the mean value of the percentage of survival ± SD of three independent replicates from at least two independent cultures, and asterisks identify significant differences at the P < 0.05 (*) or P < 0.01 (**) levels (ANOVA). (B) Growth inhibition by heavy metals in M9 minimal medium containing 0.2% (w/v) glucose or succinate as the sole C source. Experiments were performed in the presence of either 0.1 mM Cd2+ or 0.1 mM Cu2+ as indicated, and the percentages of growth inhibition were calculated at 24 h by comparing the growth of the mutants with that of P. putida KT2440. Error bars represent the mean value of the percentage of growth inhibition ± SD of three independent replicates from at least three independent cultures. All the differences in this parameter were significant (P < 0.05, ANOVA) as compared to the growth inhibition of P. putida KT2440.