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Antimicrobial Agents and Chemotherapy, July 1998, p. 1682-1688, Vol. 42, No. 7
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

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

Qixun Zhao, Xian-Zhi Li, Ramakrishnan Srikumar, and Keith Poole*

Department of Microbiology and Immunology, Queen's University, Kingston, Ontario K7L 3N6, Canada

Received 23 February 1998/Returned for modification 16 April 1998/Accepted 6 May 1998

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 mexAB-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 (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.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, Queen's University, Kingston, Ontario K7L 3N6, Canada. Phone: (613) 545-6677. Fax: (613) 545-6796. E-mail: poolek{at}post.queensu.ca.

Antimicrobial Agents and Chemotherapy, July 1998, p. 1682-1688, Vol. 42, No. 7
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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