Objectives: To investigate the resistance mechanisms of meropenem-resistant, ceftazidime-susceptible Pseudomonas aeruginosa isolates, in a clinical setting where VIM-2 or VIM-4 metallo-beta-lactamase (MBL)-producing pseudomonads are common.
Methods: During May to December 2003, 13 consecutive meropenem-resistant, ceftazidime-susceptible P. aeruginosa isolates were recovered from separate patients at the University Hospital of Larissa, Thessaly, Greece. The isolates were studied by Etest MBL, PCR for blaVIM, blaIMP and blaSPM genes and PFGE. Experiments were performed to detect synergy between meropenem or other antimicrobials and the efflux pump inhibitor carbonyl cyanide-m-chlorophenylhydrazone (CCCP). The isolates were also tested by PCR and RT-PCR for the expression of the genes mexB and mexY, which encode the efflux pumps MexAB-OprM and MexXY-OprM.
Results: Twelve of the isolates, belonging to six distinct PFGE types, gave negative results in the MBL Etest and lacked genes encoding MBLs but exhibited synergy between meropenem and CCCP, indicating that efflux pump activity contributed to the meropenem resistance. All 12 isolates were positive for mexB and 11 were also positive for mexY genes. RT-PCR showed that 10 and five isolates over-expressed mexB and mexY, respectively. One isolate was blaVIM-2-positive and did not show synergy with CCCP, or harbour mexB or mexY.
Conclusions: In our hospital, where MBL-producing P. aeruginosa were previously prevalent, meropenem resistance due to the overexpression of efflux pumps has also now emerged. Early recognition of this resistance mechanism should allow the use of alternative beta-lactams, such as ceftazidime, which would be inactive even against phenotypically susceptible MBL producers.