The role of mex-gene products in antibiotic extrusion in Pseudomonas aeruginosa.

The antibiotic extrusion machinery in Pseudomonas aeruginosa is assembled from the mex-operon encoded proteins, OprM and MexA-MexB, connecting the outer and inner membranes. To envisage the role of these proteins in antibiotic extrusion and resistance, we employed the gene replacement technique to construct mutants deficient in mexA, mexB, or oprM, and all possible combinations of these genes. Using the Southern and the Western blotting methods, we confirmed that only the target genes were disrupted. All the mutants deficient in OprM exhibited a 4 to 16 times higher susceptibility against quinolone antibiotics, chloramphenicol, and gentamicin than the parent strain. The mutants deficient in MexA or MexB or both MexA and MexB were only 2 to 4 times more susceptible to these antibiotics than the parent strain. All the mutants lacking MexA, MexB, or OprM showed stereospecific hypersusceptibility to beta-lactam antibiotics than the parent strain. However, the extent of susceptibility to each beta-lactam was comparable among the mutants. Strains lacking OprM accumulated the highest level of ciprofloxacin among all these isogenic strains. The strains lacking either MexA or MexB accumulated lower levels of ciprofloxacin than the mutant lacking OprM, but the levels were still higher than in the parent strain. The results are consistent with the antibiotic susceptibility of these strains. These results suggest that the extrusion of antibiotics occurs most efficiently with a whole assembly of MexA/B-OprM, but it remains a possibility that OprM interacts with a putative inner membrane pump(s).