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.