Interplay between Chromosomal β-Lactamase and the MexAB-OprM Efflux System in Intrinsic Resistance to β-Lactams in Pseudomonas aeruginosa

We investigated the role of chromosomal β-lactamase and the MexAB-OprM efflux system in intrinsic resistance to β-lactams in Pseudomonas aeruginosa. Determination of the susceptibilities of a series of isogenic mutants with impaired production of the β-lactamase and the efflux system to 16 β-lactams including penicillins, cephems, oxacephems, carbapenems, and a monobactam demonstrated that the intrinsic resistance of P. aeruginosa to most of the β-lactams is due to the interplay of both factors.