FIG. 6.

The roles of Rgg and PerR in stress-induced adaptation to H₂O₂ stress. (A) H₂O₂ sensitivity of GAS in PBS. Bacteria were harvested from e-stat and, after being washed, were exposed in PBS to 4 mM H₂O₂ for 2 h at 37°C. The numbers of viable bacteria were determined by plating dilution series onto THY agar. The values are mean survival percentages ± SD of six independent determinations from two independent cultures (CFU in the presence of H₂O₂ divided by the averaged CFU in the absence of H₂O₂). Student's t test: *, P < 0.05; **, P < 0.01; ***, P < 0.005. (B) Effect of PerR deficiency on the ability of GAS to establish a replicative state in the presence of H₂O₂. Bacteria from e-stat cultures were inoculated into fresh THY containing 750 μM H₂O₂. The bacteria were incubated at 37°C as standing cultures, and the OD₆₀₀s were measured at 1-hour intervals. The results from five independent cultures are expressed as mean growth yield percentages ± SD (OD₆₀₀s in the presence of H₂O₂ divided by the averaged OD₆₀₀s without H₂O₂). Part of the data is also shown in Fig. 1B. (C) H₂O₂ sensitivity of stress-adapted GAS in PBS. Bacteria were grown to exp and, after the addition of H₂O₂, t-BHP, or ethanol (pretreatment) at a sublethal concentration, the bacteria were grown to e-stat. The bacteria were harvested and, after being washed, were exposed in PBS to 4 mM H₂O₂ for 2 h at 37°C. The numbers of viable bacteria were determined by plating dilution series onto THY agar. The values are mean survival percentages ± SD of six independent determinations from two independent cultures (CFU in the presence of H₂O₂ divided by the averaged CFU in the absence of H₂O₂). Significant increase in H₂O₂ resistance, due to the stress adaptation, is indicated by asterisks (Student's t test; ***, P < 0.005).