Characterization of Ertapenem-Resistant Enterobacter cloacae in a Taiwanese University Hospital

The emergence of carbapenem resistance in Enterobacteriaceae has become a great concern. The aim of this study was to characterize ertapenem-resistant Enterobacter cloacae isolates from a Taiwanese university hospital. A total of 355 nonduplicated E. cloacae isolates collected in 2007 were analyzed by antimicrobial susceptibility testing with and without an inhibitor of efflux pumps and AmpC β-lactamase. The phenotype of extended-spectrum β-lactamase (ESBL), profile of outer membrane proteins (OMPs), and clonal relatedness were investigated by the double-disk synergy test, urea/SDS-PAGE, and pulsed-field gel electrophoresis (PFGE), respectively. β-Lactamase genes were examined by PCR and sequencing, and the expression of efflux pump gene acrB was evaluated by reverse transcription-PCR. The contribution of porin deficiency to resistance was investigated by restoring functional porin genes on plasmids. We demonstrated that ertapenem resistance was prevalent (53/355; 14.9%) in E. cloacae. Among the strains, IMP-8, SHV-12, and TEM-1 β-lactamases were identified in 3 (5.7%), 40 (75.5%), and 46 (86.8%) isolates, respectively. PFGE showed clonal diversity among these isolates. Phenotypes of ESBL, AmpC β-lactamase overproduction, an active efflux pump, and change in the expression of OMPs were found in 18 (34%), 11 (20.8%), 51 (96.2%), and 23 (43.4%) of ertapenem-resistant strains, respectively. Ertapenem MICs were restored in strains with OmpC and OmpF expression plasmids. This study suggests that ESBL, AmpC β-lactamase overproduction, and decreased OMP expression combined with an active efflux pump contribute to the ertapenem resistance of E. cloacae. The presence of IMP-8 may also play a partial role in ertapenem resistance in Taiwan.