Contribution of Outer Membrane Efflux Protein OprM to Antibiotic Resistance in Pseudomonas aeruginosa Independent of MexAB

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Abstract
A Pseudomonas aeruginosa strain carrying an insertion of an ΩHg interposon in the mexB gene (mexB::ΩHg; 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 alacZ transcriptional fusion vector, promoter activity likely responsible for OprM expression in themexB::ΩHg mutant was identified upstream ofoprM. Introduction of the oprM gene, but not the mexAB genes, into a P. aeruginosamultidrug-susceptible ΔmexAB-oprM mutant increased resistance to quinolones, cephalosporins, erythromycin, and tetracycline. A ΔmexAB-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 (in 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.