Berberine inhibits the MexXY-OprM efflux pump to reverse imipenem resistance in a clinical carbapenem-resistant Pseudomonas aeruginosa isolate in a planktonic state.

Pseudomonas (P.) aeruginosa is an ubiquitous and metabolically versatile opportunistic pathogen and may cause various life-threatening diseases. Due to increasing emergence of resistance to carbapenems, novel drugs with improved antibacterial activities compared with those of traditional antibiotics are required. In the present study, berberine (BEB), a natural isoquinoline alkaloid, was used in combination with imipenem (IMP), a commonly-used carbapenem, to investigate their antibacterial activities against a clinical P. aeruginosa isolate PA012 and the potential mechanism. Screening revealed that the minimum inhibitory concentrations (MICs) of BEB and IMP were 512 and 256 µg/ml, respectively. The combination of BEB (1/4 MIC) and IMP (1/8 MIC) exhibited a synergistic effect with a fractional inhibitory concentration index of 0.375. The synergism of BEB and IMP was also demonstrated in a time-kill test and by scanning electron microscopic observation. Treatment with BEB at ¼ MIC in combination with IMP at 1/16, 1/8, 1/4 and ½ MIC revealed a concentration-dependent promoting effect of IMP on the intracellular accumulation of BEB and inhibition of bacterial adhesion. Further analysis of gene expression revealed that BEB (1/4 MIC) combined with IMP (1/8 MIC) decreased MexZ, MexX, MexY and outer membrane protein (Opr)M by 38, 35, 46 and 49% in PA012. In conclusion, these results suggested that IMP had synergistic effects with BEB against the clinical isolate PA012 via inhibition of the MexXY-OprM efflux pump.