The hfq∆ mutant is highly sensitive to oxidative stress. Aerobic, exponentially growing cultures of MR-1/empty vector, MR-1/phfq, hfq∆ /empty vector, and hfq∆ /phfq were treated for 15 minutes with either (A and B) 0.4 mM H2O2 or (C and D) 5mM methyl viologen (paraquat) and then immediately titered. Survivorship ratios were determined by calculating the ratio of the number of viable cells in the treated cultures to the number of viable cells in mock (H2O) treated cultures. Values on the graphs are the mean survivorship ratios for three independent experiments. Error bars in (A) and (C) indicate standard deviations. The hfq∆ /empty vector survival rate is statistically different from the other three strains in both the H2O2 and methyl viologen experiments (** indicates that P < 0.005 for comparison of the hfq∆ /empty vector strain data to each of the other strains in unpaired two-tailed Student’s T-tests). Panels (B) and (D) demonstrate typical ten-fold dilution series results obtained after treatment of strains MR-1/empty vector, MR-1/phfq, hfq∆ /empty vector, and hfq∆ /phfq with (B) H2O (mock) or H2O2 or (D) H2O (mock) or methyl viologen.