Figure 1

Susceptibility to colistin can be increased by combination with CA439. Checkerboard assay showing the combination of colistin with (a) increasing doses of CA439 or (b) 25 μM of CA-analogues that do not inhibit the PhoP/PhoQ system. Dark blue regions represent higher cell density. Data represent the mean OD (600 nm) of at least three biological replicates. Right panel shows the CFU counting obtained in each growth condition: (a) DMSO (LB-medium with the addition of DMSO), CA439 (25 μM CA439 alone), colistin (0.312 μg mL⁻¹ colistin alone) and CA439 + colistin (25 μM CA439 + 0.312 μg mL⁻¹ colistin) and (b) 25 μM of each compound with or without 1.25 μg mL⁻¹ colistin. Fractional inhibitory concentration (FIC) was obtained by dividing the colistin MIC in the presence of CA439 (0.312 μg mL⁻¹) by the colistin MIC in the absence of CA439 (2.5 μg mL⁻¹). Statistical analysis was performed (a) using one-way ANOVA with Tukey’s correction multiple comparison test (****P < 0.0001; ns, no significant differences) and (b) unpaired t test for each compound assayed (ns, no significant difference).