engleski

False positive phenotypic detection of metallo-beta-lactamases in Acinetobacter baumannii

Phenotypic detection of metallo-β-lactamases (MBLs) in Acinetobacter baumannii is a serious challenge to clinical microbiologists. MBLs are inhibited by metal chelators such as EDTA. Production of MBLs cannot be recognized based on resistance phenotype. Therefore, phenotypic tests using EDTA are recommended. The aim of this study was to investigate the sensitivity and specificity of inhibitor based with EDTA for the detection of MBL . In total, 172 A. baumannii strains (123 carbapenemase positive and 49 carbapenemase negative) were analyzed. Phenotypic detection of MBLs was performed by the combined disk test with EDTA and the EPI- dilution test (EPI-DT). Both tests were positive in all 11 isolates possessing VIM-1 MBL, showing sensitivity of 100%. However, false positive results were observed in strains with class D carbapenemases using both tests - all OXA-23 and OXA-24/40 producing organisms and most OXA-58 positive strains (77% with CDT- EDTA versus 65% with EPI-DT). False positive results can occur because oxacillinases are converted to a less active state in the presence of EDTA, leading to the augmentation of the inhibition zone around the carbapenem disk or reduction of carbapenem MICs. This study showed a high sensitivity, but low specificity, of phenotypic methods in the detection of MBLs.

Acinetobacter baumannii ; metalo-beta-lactamases ; tigecyline susceptibility testing

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