Objectives: Carbapenems are the drugs of choice for the treatment of serious infections caused by ESBL- and AmpC-positive Enterobacteriaceae. An increasing trend of resistance to these antibiotics has been observed recently. The aim of the study was to determine resistance to carbapenems in clinical isolates of the Enterobacteriaceae family and its mechanism.
Methods: Between 1 April 2009 and 31 August 2010, Enterobacteriaceae were isolated from clinical samples obtained from patients hospitalized in the University Hospital Olomouc, Czech Republic (1,406 beds incl. 155 ICU beds). The strains were identified using the standard microbiological methods and their susceptibility to antibiotics was determined by the microdilution method. The identification of the isolates with the minimum inhibitory concentration (MIC) of meropenem of 2 mg/L or more was confirmed by the Phoenix automated system (Becton Dickinson). The MIC of meropenem was verified by the E-test and also Phoenix automated system. The isolates were tested for carbapenemase production using the modified Hodge test, a combined test with 3-aminophenylboronic acid (3-APB) and EDTA, the CD-test for serine carbapenemases and modified DDST (mDDST) for metallo-beta-lactamases (MBL). ESBL and AmpC production was determined by the mDDST and modified AmpC test, respectively. Genes encoding production of serine carbapenemases, MBL, bla(OXA-23), bla(OXA-48), ESBL and AmpC enzymes were detected with a set of primers that amplify specific segments of individual beta-lactamases. TEM- and SHV-positive PCR products were characterized by restriction analysis.
Results: From a total of 12,605 Enterobacteriaceae, 9 strains were isolates with the MIC of meropenem of 2 mg/L or more. Seven isolates were classified as Klebsiella pneumoniae and two as Enterobacter cloacae. The MIC of meropenem for these strains ranged from 2 mg/L to 16 mg/L. The modified Hodge test, the combined test with 3-APB and EDTA, CD-test, mDDST for MBL and a series of PCR analyses did not detect production of class A carbapenemases, MBL, OXA-23 or OXA-48 enzymes in any of the tested strains. In 6 Klebsiella pneumoniae strains and 1 Enterobacter cloacae strain, the mDDST test and genetic analysis revealed ESBL production (CTX-M and SHV types), and in 2 strains, AmpC production was detected (DHA and EBC types).
Conclusion: The prevalence of the Enterobacteriaceae with the MIC of meropenem ≥ 2 mg/L in the University Hospital Olomouc was 0.07 %. None of the strains produced either serine carbapenemases or MBL. Borderline resistance of the strains to carbapenems was determined by the ESBL and AmpC production with another associated mechanism of resistance.