Investigation of the Susceptibilities of Extended-Spectrum Beta-Lactamase Producing Escherichia coli and Klebsiella spp. Strains to Ertapenem and Other Carbapenems

Infections caused by extended-spectrum beta-lactamase (ESBL) producing Escherichia coli and Klebsiella spp. constitute severe problems. Carbapenems are commonly used to treat these infections. However, infections caused by carbapenem-resistant gram-negative bacteria show an increasing trend recently. The aim of this study was to investigate the susceptibilities of ESBL-producing E.coli and Klebsiella spp. to ertapenem and other carbapenems. A total of 239 E.coli, 28 K.pneumoniae and 11 K.oxytoca strains isolated from clinical specimens (208 urine, 16 blood, 26 wound, 17 sterile body fluids, four tracheal aspirates and seven others) of hospitalized patients and outpatients between January 2007-February 2008, were included to the study. The isolates were identified by conventional methods, and antibiotic susceptibility tests were performed by Kirby Bauer disc diffusion method according to Clinical and Laboratory Standards Institute (CLSI) standards. ESBL production was tested by double disk diffusion method. When ESBL production was indeterminate, cefotaxime-clavulanic acid E test (BioMerieux, France) was used. According to the CLSI standards modified Hodge test was performed for carbapenem resistant isolates and minimal inhibitory concentration (MIC) values were detected for ertapenem (Etest (R), BioMerieux, France), imipenem and meropenem (M.I.C. Evaluator Strips, Oxoid, UK). All of the isolates were found susceptible to amikacin (278/278; 100%), whereas the susceptibility rates for imipenem/meropenem and ertapenem were 99.3% (276/278) and 98.6% (274/278), respectively. When evaluated individually, ertapenem susceptibilities of E.coli, K.pneumoniae and K.oxytoca strains were 99.2%, 96.4% and 90.9%, respectively, while these rates were 100%, 96.4% and 90.9%, respectively, for imipenem/meropenem. Carbapenem resistance was detected in two E.coli, one K.oxytoca and one K.pneumoniae isolates. While two Klebsiella spp. isolates were resistant to all of the tested carbapenems (MIC > 32 mu g/ml), two E.coli isolates were resistant to ertapenem (MIC > 32 mu g/ml) but susceptible to imipenem (MIC= 0.25 mu g/ml) and meropenem (MIC= 0.5 mu g/ml). Carbapenemase production was demonstrated by modified Hodge test in all of the carbapenem-resistant isolates. In conclusion, ESBL-producing gram-negative isolates should be routinely tested with a screening method for carbapenemase activity and confirmation tests should be performed in suspected cases.