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 interposon in the mexB gene (mexB::Omega Hg; strain K879) produced markedly reduced but still detectable levels of OprM, the product of the third gene of the mexAB-oprM multidrug efflux operon. By using a lacZ transcriptional 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 increased resistance to quinolones, cephalosporins, erythromycin, and tetracycline. A Delta mexAB-oprM strain carrying the oprM gene accumulated markedly less antibiotic than the deletion strain without oprM. Antibiotic accumulation by the MexAB(-) OprM(+) strain was markedly enhanced upon treatment of cells with the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), indicating that MexAB-independent OprM function likely involves an efflux process. Moreover, pretreatment of cells with CCCP prior to the accumulation assay abrogated any differences in accumulation levels between the MexAB(-) OprM(+) and MexAB(-) OprM(-) strains, indicating that reduced drug accumulation by the OprM(+) strain tin the absence of CCCP) cannot be due to OprM-mediated reduction in outer membrane permeability. It appears, therefore, that OprM can be expressed and function in a drug efflux capacity independent of MexAB.