Twenty-five isolates of Pseudomonas aeruginosa with different meropenem susceptibilities were subjected to quantitative RT-PCR for analysis of transcription levels of oprD, mexB and mexD, and, in selected isolates, PA3720, which is hyper-expressed in nalC efflux mutants. Regulator genes of efflux pump MexAB-OprM, mexR and PA3721 (putative) were sequenced in selected isolates. The potential for mathematical reconstruction of the ideal susceptible population using normalised resistance interpretation (NRI) was also studied. In three isolates with intermediate susceptibility to meropenem (according to Swedish breakpoints), a reduction in MIC from 4 to 2 mg/L was observed with efflux inhibitor MC-207,110. These isolates would be considered susceptible according to British Society for Antimicrobial Chemotherapy and NCCLS breakpoints. These three isolates had between 4.6- and 5.0-fold increases in mexB transcription. None of these isolates had significant nalB mutations, but an Ala145-->Val mutation was observed in PA3721 in two of the isolates. However, these isolates had moderately increased production of PA3720 only. Single-strain regression analysis did not detect any major biological differences between the different groups. Using NRI, a disk-diffusion susceptibility breakpoint of >/= 28 mm was generated. Isolates with intermediate susceptibility to meropenem, which are considered fully susceptible in many countries, displayed possible low-grade meropenem resistance mechanisms, implying that the susceptibility breakpoint should be reconsidered. The increased transcription of mexB mRNA in such isolates seems unrelated to nalB or nalC mutations.

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