Effect of an Efflux Pump Inhibitor on the Function of the Multidrug Efflux Pump CmeABC and Antimicrobial Resistance in Campylobacter

CmeABC, a multidrug efflux pump, contributes to the resistance of Campylobacter to a broad range of antimicrobials. We hypothesize that an efflux pump inhibitor (EPI) may inhibit the function of CmeABC and control antibiotic resistance in Campylobacter. In this study, we examined the effect of EPI Phe-Arg β-naphthyl-amide dihydrochloride (MC-207,110) on the susceptibility of Campylobacter to various antimicrobials. The presence of the EPI resulted in a 2- to 2048-fold reduction in the minimum inhibitory concentration (MIC) of antimicrobials known to be substrates of the CmeABC pump in all Campylobacter strains. Both intrinsic and acquired resistance of C. jejuni to erythromycin was decreased drastically (64- to 128-fold reduction in the MIC) in the presence of the EPI while the MICs of fluoroquinolones were only slightly decreased (2- to 4-fold). Examination of 57 Campylobacter isolates from various origins further demonstrated that MC-207,110 decreased the MICs of erythromycin (2- to 512-fold) in all isolates. Compared to wild-type strains, the isogenic CmeB mutants displayed smaller magnitudes of reduction in the MICs of antimicrobials in the presence of the EPI, indicating the inhibitory effect of the EPI is primarily CmeABC-dependent. The inhibitory effect of MC-207,110 was also dose-dependent, and as little as 0.5 µg/mL of the EPI resulted in a decreased MIC for erythromycin in C. jejuni. More importantly, the presence of MC-207,110 decreased the frequency of emergence of erythromycin-resistant mutants in C. jejuni (<10⁻¹¹, well below the normal frequency of ∼10⁻⁸). Together, these findings indicate that EPI MC-207,110 inhibits the function of CmeABC efflux pump and potentiates the activity of antibiotics against Campylobacter. Inhibition of CmeABC by EPI is a promising approach in combating antibiotic resistance of Campylobacter in humans and animal reservoirs.