Performance of four different agar plate methods for rectal swabs, synergy disk tests and metallo-β-lactamase Etest for clinical isolates in detecting carbapenemase-producingKlebsiella pneumoniae

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The aims of the study were to compare four different agar plate methods in the identification of carbapenemase-producing Klebsiella pneumoniae (CP-Kp) from rectal samples and to assess the role of phenotypic methodologies in the identification of carbapenemase type from clinical K. pneumoniae isolates. Two chromogenic agars (Brilliance CRE and CHROMagar KPC) were compared to MacConkey agar plates with ertapenem (ERT) or imipenem (IMP) disks for the identification of CP-Kp from 912 rectal swabs. CP-Kp was detected in 329 samples by either agar methodology (299 K. pneumoniae carbapenemase positive, 27 Verona integron-encoded metallo-β-lactamase positive and 3 K. pneumoniae carbapenemase and Verona integron-encodedmetallo-β-lactamase positive). Sensitivity of Brilliance CRE, CHROMagar KPC and MacConkey agar plus IMP or ERT disk (inhibition zone <25 mm) was 96.8, 99.2, 67.2 and 81.8 %, while specificity was 90.9, 78.2, 98.1 and 97.9 %, respectively. Synergy meropenem-disk tests with EDTA or phenylboronic acid were used in order to detect the carbapenemase type as compared to PCR results (blaVIM, blaKPC and blaNDM) from 2515 isolates with reduced susceptibility to any of the Etest-examined carbapenems (ERT, IMP or meropenem). Metallo-β-lactamase MP/MPI Etest was applied in 616 isolates. Sensitivity was 98.4, 90.9 and 82.2 % for phenylboronic acid synergy test, EDTA synergy test and metallo-β-lactamase Etest, respectively, while their specificity was high (>97.5 %). Phenotypic methodologies can provide reliable results for the identification of carbapenemase production among K. pneumoniae isolates. Chromogenic agars can be applied in high-risk patients as part of surveillance and infection control programs.

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