Background:

Resistance of Pseudomonas aeruginosa (PA) to anti-pseudomonal carbapenems is usually due to the combination of low intracellular concentrations due to OprD loss and efflux pumps and slow hydrolysis by beta-lactamases. For MER resistance to arise, both OprD loss and efflux pump expression is required, while IMP is affected only by OprD. Therefore it has been hypothesised that the two mechanisms required for MER resistance will result in lower likelihood of emergence of resistance.

Methods:

10 PA clinical isolates and one ATCC strain were studied. About 5 × 10⁴ CFU/well were inoculated onto 96 multi-well plates containing Mueller–Hinton broth (MHB) serial dilutions of IMP and MER. Isolates were serially transferred. Number of transfers in sub-MIC concentration until emergence of resistance was determined. MICs were determined according to NCCLS guidelines. OMPs mapped on SDS-PAGE.

Results:

Each of the selected isolates had distinct PFGE pattern. All had MIC <2 for ciprofloxacin, two were resistant to ceftazidime (CTZ). The MIC50 for IMP was 2 (range 1–4) and for MER 1 (range 0.25–2). Emergence of resistance was detected after two to eight passages and occurred at similar rate for both IMP and MER (average 4.6 vs. 4.8 passages). In each isolate resistance to both agents occurred at the same pace (up to one passage difference). At the time IMP resistance emerged, CTZ MICs did not change in seven strains and increased two to five dilutions in three strains. On emergence of MER resistance, CTZ MICs did not change in five isolates and increased two to four dilutions for the other five. Only one of these isolates became resistant to CTZ. Over-2-dilution-increase in ciprofloxacin or tetracyclin MIC occurred in one isolate at emergence of resistance to IMP and in six at emergence to MER. OMPs and beta lactamases activity correlates were determined.

Conclusions:

We found similar pace of emergence of MER resistance compared with IMP in PA, despite the lower MIC, and the additional mechanism required for its development. The loss of OprD by both agents and induction of efflux pumps by MER were shown.