Carbapenem resistance in ESBL positive Enterobacteriaceae isolates causing invasive infections

The aim of this study was to investigate the presence of carbapenem resistance in Enterobacteriaeceae isolates recovered from invasive infections, in Hacettepe University Hospital, Ankara, Turkey, between 2005-2009, by phenotypic and genotypic methods. A total of 210 non-duplicated Escherichia coli (n= 153), Klebsiella pneumoniae (n= 47) and Klebsiella oxytoca (n= 10) isolates which were all determined to be extended-spectrum beta-lactamase (ESBL) positive with the BD Phoenix automated identification and antibiotic susceptibility system (Sparks, USA), were included in the study. The isolates were recovered from patients with bloodstream infections. Susceptibility of the isolates to imipenem, meropenem and ertapenem was detected with microdilution method according to the standards of Clinical and Laboratory Standards Institute (CLSI) minimal inhibitory concentration (MIC) breakpoints. Doripenem susceptibility was detected by the E-test (bioMerieux, Hazelwood, USA). All isolates which were found to be non-susceptible to any of the carbapenem antibiotics tested, were characterized by the phenotypic confirmatory tests and the presence of the resistance genes; blaAmpC, blaCTX-M, blaKPC, blaNDM, blaOXA, blaIMP ve blaVIM were screened by polymerase chain reaction (PCR). Among the 210 ESBL-producing Enterobacteriaceae blood isolates, 23 (11%) were identified as non-susceptible to any of the carbapenems tested. Resistance rates for imipenem, meropenem and ertapenem were 5.7% (n= 12), 1.9% (n= 4) and 2.4% (n= 5), respectively. Doripenem was more active than the other carbapenems, with a resistance rate of 1.0%. Seven of 23 isolates were ESBL negative with cefotaxime/clavulanic acid (CTX/CLA) and ceftazidime/clavulanic acid (CAZ/CLA) combined disk diffusion test, however, six of them were ESBL positive with the addition of boronic acid (BA) to CTX/CLA. Among the three isolates positive for Modifiye Hodge test (MHT) and/or ertapenem-BA tests, blaOXA-48 was detected in one and blaAmpC in the other. Phenotypic pAmpC activity was present in three K.pneumoniae isolates of which one was positive for blaAmpC gene. One K.pneumoniae isolate resistant to all carbapenems with MICs > 256 µg/ml and positive for phenotypic meropenem-BA, MHT, imipenem-EDTA, ceftazidime-CAZ/CLA, cefoxitin-BA production, was found to inhabit blaOXA-48 gene. Five isolates were positive for blaOXA-1 and one for blaOXA-10. Two isolates were positive for blaCTX-M, however blaIMP, blaVIM and blaNDM-1 gene