CONTRIBUTION OF OUTER-MEMBRANE EFFLUX PROTEIN OPRM TO ANTIBIOTIC-RESISTANCE IN PSEUDOMONAS-AERUGINOSA INDEPENDENT OF MEXAB

A Pseudomonas aeruginosa strain carrying an insertion of an Omega Hg i nterposon in the mexB gene (mexB::Omega Hg; strain K879) produced mark edly reduced but still detectable levels of OprM, the product of the t hird gene of the mexAB-oprM multidrug efflux operon. By using a lacZ t ranscriptional fusion vector, promoter activity likely responsible for OprM expression in the mexB::Omega Hg mutant was identified upstream of oprM Introduction of the oprM gene, but not the mexAB genes, into a P. aeruginosa multidrug-susceptible Delta mex-AB-oprM mutant increase d resistance to quinolones, cephalosporins, erythromycin, and tetracyc line. A Delta mexAB-oprM strain carrying the oprM gene accumulated mar kedly less antibiotic than the deletion strain without oprM. Antibioti c accumulation by the MexAB(-) OprM(+) strain was markedly enhanced up on treatment of cells with the uncoupler carbonyl cyanide m-chlorophen ylhydrazone (CCCP), indicating that MexAB-independent OprM function li kely involves an efflux process. Moreover, pretreatment of cells with CCCP prior to the accumulation assay abrogated any differences in accu mulation levels between the MexAB(-) OprM(+) and MexAB(-) OprM(-) stra ins, indicating that reduced drug accumulation by the OprM(+) strain t in the absence of CCCP) cannot be due to OprM-mediated reduction in ou ter membrane permeability. It appears, therefore, that OprM can be exp ressed and function in a drug efflux capacity independent of MexAB.