Proton motive force dependence of mtrC- and cpxA-mediated AP resistance. Strains 35000HP, 35000HPmtrC, and 35000HPΔcpxA were pretreated with DMSO alone or CCCP in DMSO and challenged with LL-37 (2 μg/ml) (A) or HBD-3 (10 μg/ml) (B). The data represent the means and standard errors of 6 independent assays. The single asterisks indicate significant reductions in sensitivity compared with 35000HP (P < 0.03). The double asterisks indicate significant reductions in AP sensitivity with or without CCCP treatment for each strain (panel A, P < 0.001; panel B, P < 0.04). ns, not significant (P ≥ 0.05).