Figure 5.

PhoPQ is required for Shigella to resist an environment of low Mg²⁺, acidic pH, or the presence of polymyxin B. Sf301, ΔphoPQ and ΔphoPQc were grown in N minimal medium supplemented with 10 mM MgCl₂ (A) or 10 μM MgCl₂ (B). The OD₆₀₀ was determined every 60 min for 14 h. Sf301, ΔphoPQ and ΔphoPQc were grown in E glucose broth at pH 7 (C) or pH 5 (D). The OD₆₀₀ was determined every 60 min for 14 h. Sf301, ΔphoPQ and ΔphoPQc were diluted in sterile 0.85% saline to about 5 × 10³ cells per ml, and exposed for 1 h at 37°C to various concentrations of polymyxin B. Survivors were measured by plating on LB. The survival rate was calculated as the number of bacteria treated with polymyxin B divided by that of the untreated control (E). Results are mean from three independent assays performed in duplicate. Error bars correspond to standard deviations.