FIG 10.

Concentration-dependent tobramycin killing of washed stationary-phase cells. (A) Washed stationary-phase cultures of wild-type PA14 and ΔndvB and ΔrpoS strains were exposed to various tobramycin (TOB) concentrations (0 to 500 μg/ml) for 8 h at 37°C. Cell survival following antibiotic treatment was assessed by colony counts using the drop plate method. Data are presented as means ± SEMs for three independent experiments. In some cases, error bars are smaller than the symbols. Statistical significance was determined by two-way ANOVA with Dunnett's post hoc multiple-comparison test for each concentration of tobramycin (PA14 was used as the comparison control). (B) Log₁₀ reduction of PA14, ΔndvB, and ΔrpoS stationary-phase cells exposed to 200 μg/ml of tobramycin (calculated as the difference between survival at 0 μg/ml and survival at 200 μg/ml from panel A). Data are means + SEMs for three experiments. Statistical significance was determined by one-way ANOVA with Dunnett's multiple-comparison test (PA14 was used as the comparison control). (C) Washed stationary-phase PA14 or ΔrpoS cells carrying pUCP19 or pUCP19::rpoS⁺ were exposed to 0 or 200 μg/ml of tobramycin. Data are means + SEMs for four independent experiments. Statistical significance was determined by two-way ANOVA with Tukey's multiple-comparison test. (D) Log₁₀ reduction of PA14 and ΔrpoS strains carrying pUCP19 or pUCP19::rpoS⁺ when exposed to 200 μg/ml of tobramycin (calculated as the difference between survival at 0 μg/ml and survival at 200 μg/ml from panel C). Data are means + SEMs for four experiments. Statistical significance was determined by one-way ANOVA with Tukey's multiple-comparison test.