Sensitive and specific phenotypic assay for metallo-beta-lactamase detection in Enterobacteria by use of a moxalactam disk supplemented with EDTA

FALSE POSITIVE PHENOTYPIC DETECTION OF METALLO-BETA-LACTAMASES IN ACINETOBACTER BAUMANNII

Phenotypic detection of metallo-β-lactamases (MBLs) in Acinetobacter (A.) baumannii is a serious challenge to clinical microbiologists. MBLs are inhibited by metal chelators such as ethylenediaminetetraacetic acid) (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 tests (EDTA) for detection of MBL. A total of 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 (CDT-EDTA) and EPI-dilution test (EPI-DT). Both tests were positive in all 11 isolates possessing VIM-1 MBL, showing 100% sensitivity. However, false positive results were observed in strains with class D carbapenemases using both tests, i.e. all OXA-23 and OXA-24/40 producing organisms and most OXA-58 positive strains (77% with CDT-EDTA vs. 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 augmentation of the inhibition zone around the carbapenem disk or reduction of carbapenem minimum inhibitory concentrations. This study showed high sensitivity but low specificity of phenotypic methods in the detection of MBLs.

Current methods for the identification of carbapenemases

Detection of carbapenemases in clinical microbiology labs is a challenging issue. Comparison of the results of susceptibility testing with the breakpoint values of carbapenems is the first step in the screening of carbapenemase producers. To date, screening of carbapenemase-producing (CP) bacteria has been mostly performed by a selective medium. Although these media are practical for the detection of most CP isolates, the inoculated plates have to be incubated overnight. Subsequently, we need the confirmation of the carbapenemase producers present in the culture medium by additional testing [e.g. inhibition studies with liquid or solid media, modified Hodge test (MHT), or gradient strips], which can take up to another 48 hours. Despite the lack of discrimination between the three different classes of carbapenemases (KPC, MBL and OXA) and difficulties in the interpretation of the results, the MHT is usually deemed as the phenotypic reference method for the confirmation of carbapenemase production. Molecular techniques, such as real-time polymerase chain reaction (PCR) assays, in contrast to phenotypic methods that are very time consuming, are faster and allow for the quick identification of carbapenemase genes. These techniques can detect and characterize carbapenemases, including NDM- and KPC-mediated resistance, which is critical for epidemiological investigations. The aim of this review is to gather a summary of the available methods for carbapenemase detection and describe the strengths and weaknesses of each method.

Phenotypic screening of carbapenemases and associated β-lactamases in carbapenem-resistant Enterobacteriaceae

Dissemination of carbapenem resistance among Enterobacteriaceae poses a considerable threat to public health. Carbapenemase gene detection by molecular methods is the gold standard but is available in only a few laboratories. The aim of this study was to test phenotypic methods for the detection of metallo-β-lactamase (MBL)- or Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae and associated mechanisms of β-lactam resistance against a panel of 30 genotypically characterized carbapenem-resistant Enterobacteriaceae : 9 MBL, 7 KPC, 6 OXA-48, and 8 extended-spectrum β-lactamase (ESBL) or AmpC β-lactamases associated with decreased permeability. We used carbapenemase inhibitor-impregnated agar to test for carbapenem-resistant strains. Differences in the inhibition zone sizes of the meropenem, imipenem, ertapenem, and doripenem disks were measured between control and inhibitor (EDTA or phenylboronic acid [PBA] with or without cloxacillin)-impregnated Mueller-Hinton agar with a cutoff of 10 mm. All 9 MBL- and 7 KPC-producing Enterobacteriaceae were identified from the differences in zone size in the presence and absence of specific inhibitors, regardless of the carbapenem MICs and including isolates with low-level resistance to carbapenems. We also detected their associated β-lactam resistance mechanisms (11 ESBL-type and 5 class A β-lactamase 2b). No differences in zone size were observed for OXA-48-producing strains or other carbapenem resistance mechanisms such as ESBL and decreased permeability. We propose a new strategy to detect carbapenemases (MBL- and KPC-type) and associated mechanisms of β-lactam resistance (ESBL or class A β-lactamase 2b) by the use of inhibitor-impregnated agar. A rapid phenotypic detection of resistance mechanisms is important for epidemiological purposes and for limiting the spread of resistant strains by implementing specific infection control measures.