Two Efflux Systems Expressed Simultaneously in Multidrug-Resistant Pseudomonas aeruginosa

Simultaneous overexpression of MexAB-OprM and MexEF-OprN was demonstrated for a multiply antibiotic-resistant clinical isolate of Pseudomonas aeruginosa (G49). G49 also had decreased expression of OprF. No mutations in mexR or its upstream promoter region, mexT, oprM, oprF, or sigX were revealed, suggesting regulation by a hitherto undescribed locus.