Involvement of the MexXY-OprM efflux system in emergence of cefepime resistance in clinical strains of Pseudomonas aeruginosa

Cefepime (FEP) and ceftazidime (CAZ) are potent R-lactam antibiotics with similar MICs (I to 2 mu g/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 (FEPr/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 P-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 FEPr/CAZ(S) phenotype. The six isolates of this series that exhibited wild-type levels of the mexY gene were found to produce beta-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.