Bacteriostatic and bactericidal activities of eight fluoroquinolones against MexAB-OprM-overproducing clinical strains of Pseudomonas aeruginosa

Abstract

To assess the impact of stable overproduction of efflux system MexAB-OprM on the bacteriostatic and bactericidal activities of fluoroquinolones against clinical Pseudomonas aeruginosa strains.The minimal inhibitory concentrations (MICs) and the minimal bactericidal concentrations (MBCs) of eight fluoroquinolones (pefloxacin, norfloxacin, ofloxacin, moxifloxacin, levofloxacin, ciprofloxacin, trovafloxacin and grepafloxacin) were determined for nine post-therapy resistant isolates of P. aeruginosa overexpressing MexAB-OprM. Clinical significance of low-level resistance conferred by the efflux mechanism was evaluated with a Monte Carlo simulation.Compared with their pre-therapy susceptible counterparts, seven out of the nine post-therapy efflux mutants exhibited a modest two- to eight-fold increase in resistance to all the fluoroquinolones tested. Interestingly, stronger variations in resistance (up to 64-fold) were observed in two other mutants, one of which had acquired a GyrB target mutation in addition to efflux under chemotherapy. Time-kill experiments showed that MexAB-OprM up-regulation did not confer tolerance to fluoroquinolones as the ratio of MBC to MIC was less than 4 for most of the strains. To gain an insight into the clinical significance of resistance conferred by MexAB-OprM, a Monte Carlo simulation was conducted with various fluoroquinolone regimens. With this model, low levels of resistance to ciprofloxacin (MIC > or = 0.25 mg/L) or levofloxacin (MIC > or = 1 mg/L), such as those due to overproduced MexAB-OprM, were predicted to result in poor clinical outcomes.Altogether, these data strongly suggest that when derepressed, MexAB-OprM provides P. aeruginosa with a resistance that may be sufficient to impair the efficacy of single therapy with highly potent fluoroquinolones, such as ciprofloxacin and ofloxacin.