Effect of 1-(1-naphthylmethyl)-piperazine, a novel putative efflux pump inhibitor, on antimicrobial drug susceptibility in clinical isolates of Escherichia coli

Objectives: 1-(1-Naphthylmethyl)-piperazine (NMP) has been shown to reverse multidrug resistance (MDR) in Escherichia coli overexpressing resistance-nodulation-cell division type efflux pumps, but there is no data on its activity in clinical isolates of E. coli. Methods: The antimicrobial susceptibility of 60 clinical isolates of E. coli to a variety of antimicrobial agents was determined in the absence and presence of NMP and, for comparison, of Phe-Arg-beta-naphthylamide (PA beta N), another putative efflux pump inhibitor (EPI). The intracellular accumulation of ethidium bromide was measured to confirm efflux pump inhibition as the likely mechanism of action of NMP. Results: Based on a 4-fold or greater reduction of the MIC after the addition of NMP in > 50% of the isolates, significant effects of NMP at a concentration of 100 mg/L were seen for levofloxacin, linezolid and ethidium bromide. The ethidium bromide MIC changes after NMP addition correlated with differences in the ethidium bromide intracellular accumulation as measured by fluorometry in whole cell accumulation experiments. The activity of PA beta N was different from that of NMP, in particular regarding macrolide resistance reversal, suggesting different modes of action of the two putative EPIs. Conclusions: NMP is moderately active in reversing MDR in clinical isolates of E. coli and can partially restore fluoroquinolone susceptibility through inhibition of efflux pumps.