FIGURE 4.

Efflux pump inhibitor reserpine increased the susceptibility of K. pneumoniae, but not the ICEKp knockout, to antimicrobials. Addition of reserpine (RP, 50 μg/ml) significantly increased the diameter of the zone of inhibition of study strain KpRR2 and the plasmid-complemented mutant ΔICE + pSXPQA for trimethoprim (A) and tetracycline (B). By contrast, the presence (+) or absence (-) of reserpine had no significant effect on the zones of inhibition of the knockout strain ΔICE. Trimethoprim was used at 5 μg per disk and tetracycline at 25 μg per disk. Data are the mean and standard deviation of three replicates. *indicates p < 0.05 using Student’s t test. “ns” indicates p > 0.05. The assay was also performed with other antimicrobials giving similar results (Supplementary Figure 1).