Effects of oxyR gene disruption on bacterial sensitivity to reagent H2O2 and to neutrophil microbicidal systems. (a) H2O2 susceptibility of the parent strain (I, ATCC 11775), an oxyRS mutant (II, ΔoxyRS), and the mutant strain complemented with an oxyR-containing plasmid described in Methods (III, pAQ5). Mueller-Hinton agar was swabbed with a suspension of the appropriate strain, and filter-paper discs were impregnated with 10 μl of 1 mM H2O2 (10 μmol) and allowed to grow overnight at 37°C. Zones of growth inhibition were highly reproducible and stable for days, suggesting a microbicidal effect of the peroxide. (b) Bacterial strains described in a were incubated at a concentration of approximately 30 × 106 bacteria/ml, with 5 × 106 neutrophils/ml under conditions otherwise described in Table 2. At indicated times, samples were obtained to determine microbial viability by streaking dilutions of a neutrophil lysate onto Mueller-Hinton agar. Results are shown as the geometric means ± SE of data from three experiments.