Involvement of the MexXY-OprM Efflux System in Emergence of Cefepime Resistance in Clinical Strains of Pseudomonas aeruginosa

Abstract

Cefepime (FEP) and ceftazidime (CAZ) are potent β-lactam antibiotics with similar MICs (1 to 2 μg/ml) for wild-type strains of Pseudomonas aeruginosa . However, recent epidemiological studies have highlighted the occurrence of isolates more resistant to FEP than to CAZ (FEP ^r /CAZ ^s profile). We thus investigated the mechanisms conferring such a phenotype in 38 clonally unrelated strains collected in two French teaching hospitals. Most of the bacteria ( n = 32; 84%) appeared to stably overexpress the mexY gene, which codes for the RND transporter of the multidrug efflux system MexXY-OprM. MexXY up-regulation was the sole FEP resistance mechanism identified ( n = 12) or was associated with increased levels of pump MexAB-OprM ( n = 5) or MexJK ( n = 2), synthesis of secondary β-lactamase PSE-1 ( n = 10), derepression of cephalosporinase AmpC ( n = 1), coexpression of both OXA-35 and MexJK ( n = 1), or production of both PSE-1 and MexAB-OprM ( n = 1). Down-regulation of the mexXY operon in seven selected strains by the plasmid-borne repressor gene mexZ decreased FEP resistance from two- to eightfold, thereby demonstrating the significant contribution of MexXY-OprM to the FEP ^r /CAZ ^s phenotype. The six isolates of this series that exhibited wild-type levels of the mexY gene were found to produce β-lactamase PSE-1 ( n = 1), OXA-35 ( n = 4), or both PSE-1 and OXA-35 ( n = 1). Altogether, these data provide evidence that MexXY-OprM plays a major role in the development of FEP resistance among clinical strains of P. aeruginosa .