Heteroresistance contributes to variable carbapenem susceptibility levels in VIM1producing Klebsiella pneumoniae strains belonging to the same clone: consequences for routine susceptibility testing

Abstract number: P2069

Tato M., Morosini M.I., García L., Albertí S., Cantón R.

Introduction: Resistance to carbapenems due to MBL production in Enterobacteriaceae is increasingly recognized. Different levels of resistance to carbapenems have been observed in these isolates and reproducible susceptibility testing results within the same or among different methods is not always obtained. We studied the carbapenems susceptibility profiles of VIM-1 producing K. pneumoniae isolates belonging to the same clone which was involved in an epidemic in our hospital (2005–2008) affecting 18 patients.

Methods: Eighteen VIM-1 producing K. pneumoniae isolates belonging to an epidemic clone (XbaI-PFGE) were studied. Carbapenems MICs obtained by microdilution (CLSI), WIDER semiautomatic system, and Etest were compared. Errors in the clinical interpretive categories were determined considering microdilution as reference. Other mechanisms contributing to carbapenem resistance, such as porin expression, were also studied. Heteroresistance was determined by population analysis profile (PAP) in 4 selected strains displaying different imipenem MICs.

Results: Imipenem MICs were in the range of 8–>128 mg/L by microdilution, geqslant R: gt-or-equal, slanted1–>8 mg/L by WIDER and 0.75–>32 mg/L by Etest. Ranges for meropenem and ertapenem with Etest were 0.38–>32 and 0.75–>32 mg/L, respectively. Only one isolate with high level imipenem resistance (MIC > 128 mg/L) did not express porins OmpK36. High error rates were observed with WIDER and Etest when compared with microdilution. The category agreement was 28% for WIDER and 11% for Etest with 28% of very mayor errors in both cases. Low reproducibility of MICs was observed with Etest, even when the same inoculum was used (up to 4-fold dilutions of diference). Heteroresistance for imipenem was initially suspected due to the presence of colonies in the inhibition zone of Etest strips. This was confirmed by PAP results obtained in all selected strains with the exception of the porin deficient K. pneumoniae isolate that homogeneously expressed carbapenem resistance.

Conclusions: Low reproducibility of MIC results to carbapenems obtained by different susceptibility testing methods could be due to the presence of resistant subpopulations in carbapenemase producing enterobacterial isolates. Variable expression of resistance mechanisms affecting carbapenems might also contribute to this effect. Carbapenem MICs are not good markers of MBL production and reliable phenotypic methods are needed to confirm the presence of this mechanism.

Session Details

Date: 16/05/2009
Time: 00:00-00:00
Session name: 19th European Congress of Clinical Microbiology and Infectious Diseases
Location: Helsinki, Finland, 16 - 19 May 2009
Presentation type:
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