FIG 6.

Pyocyanin confers resistance to Ag⁺ toxicity by P. aeruginosa. Suspensions of the PA14 and ΔphzM strains containing equivalent numbers of CFU were exposed to serial dilutions of AgNO₃ (0 to 40 μg/ml) for 24 h. The surviving bacteria were transferred to MHA plates, the number of CFU was counted, and the percent survival was determined. (A) The MIC for the PA14 strain was 20 μg/ml. (B) In contrast, the MIC for the ΔphzM strain was 5 μg/ml. (C) Removal of pyocyanin from the PA14 strain by centrifugation and resuspension in fresh MHB (devoid of pyocyanin [− Pyo]) resulted in increased sensitivity to Ag⁺. (D) The addition of pyocyanin (20 μM) to the ΔphzM strain increased the MIC to 20 μg/ml. Data represent means plus standard deviations from five independent experiments> Values that are significantly different (P < 0.001) from the value for no AgNO₃ by ANOVA with Dunnett's multiple-comparison test are indicated by an asterisk.