Figure 6

CA439-colistin potentiation is partially Rcs-dependent. Checkerboard assay showing CA439-colistin combination in (a) rcsB mutant or (b) phoP rcsB double mutant. Dark blue regions represent higher cell density. Data represent the mean OD (600 nm) of at least three biological replicates. Right panel in shows the CFU counting obtained in each growth condition: DMSO (LB-medium with the addition of DMSO), CA439 (25 μM CA439 alone), colistin (0.156 μg mL⁻¹ for ΔrcsB and 0.078 μg mL⁻¹ for ΔphoP ΔrcsB) and CA439 + colistin (25 μM CA439 + 0.156 μg mL⁻¹ colistin for ΔrcsB and 0.078 μg mL⁻¹ colistin for ΔphoP ΔrcsB). Fractional inhibitory concentration (FIC) was obtained by dividing the colistin MIC in the presence of CA439 (0.156 μg mL^-1 for both strains) by the colistin MIC in the absence of CA439 (0.625 μg mL⁻¹ for ΔrcsB and 0.156 μg mL⁻¹ for ΔphoP ΔrcsB). Statistical analysis was performed using one-way ANOVA with Tukey’s correction multiple comparison test (****P < 0.0001; **P < 0.001; *P < 0.01; ns, no significant differences).