Outbreak of multiple clones of imipenem-resistant Acinetobacter baumannii isolates expressing OXA-58 carbapenemase in an intensive care unit

Objectives: To investigate the resistance mechanisms and the genetic relationship of imipenem-resistant Acinetobacter baumannii isolates recovered in the intensive care unit (ICU) of a tertiary care hospital. Methods: Imipenem-resistant A. baumannii clinical and environmental isolates were collected in the ICU of the Red Cross General Hospital, Athens, Greece between March and October 2002. The isolates were tested by Etest MBL, PCR, RT-PCR and sequencing for carbapenemase-encoding genes, PFGE and synergy experiments using meropenem and the efflux pump inhibitor carbonyl cyanide chlorophenylhydrazone. Results: During the study period, 15 clinical and two environmental imipenem-resistant (MIC 8 to > 128 mg/L) A. baumannii isolates were recovered. PFGE showed six different clones that included both clinical and environmental isolates. All 17 isolates were negative by Etest MBL and PCR for genes bla(IMP), bla(VIM), bla(SPM), bla(OXA-23-like) and bla(OXA-24-like). Genes bla(OXA-51-like) and bla(OXA-58-like) were amplified from 15 and 14 isolates, respectively. Sequencing of bla(OXA-51-like) amplicons identified bla(OXA-66) (nine cases) and bla(OXA-69) (six cases), whereas bla(OXA-58-like) sequences were classical bla(OXA-58). Reverse transcriptase-PCR showed that bla(OXA-51-like) genes were expressed in 12 and bla(OXA-58) in 10 isolates; in these isolates, inhibition of OXA enzymes by 200 mM of NaCl reduced carbapenem MICs by up to 4-fold. Overexpression of proton-gradient dependent efflux pumps did not contribute to carbapenem resistance in any isolate. Similarly, although AmpC expression was demonstrated in eight isolates, inhibition of AmpC with cloxacillin did not reduce the MICs of carbapenems significantly. Conclusions: These findings indicate wide dissemination of OXA-58 carbapenemase, which contributes, at least partially, to the imipenem resistance of unrelated A. baumannii isolates in our ICU.