Resistance to β-lactam antibiotics in Pseudomonas aeruginosa due to interplay between the MexAB-OprM efflux pump and β-lactamase

We evaluated the roles of the MexAB-OprM efflux pump and β-lactamase in β-lactam resistance in Pseudomonas aeruginosa by constructing OprM- deficient, OprM basal level, and OprM fully expressed mutants from β- lactamase-negative, -inducible, and -overexpressed strains. We conclude that, with the notable exception of imipenem, the MexAB-OprM pump contributes significantly to β-lactam resistance in both β-lactamase-negative and β- lactamase-inducible strains, while the contribution of the MexAB-OprM efflux system is negligible in strains with overexpressed β-lactamase. Overexpression of the efflux pump alone contributes to the high level of β- lactam resistance in the absence of β-lactamase.