Evaluation of different laboratory tests for the detection of metallo-β-lactamase production in Enterobacteriaceae

Objectives: Clinical isolates of Klebsiella pneumoniae (91), Escherichia coli (49), Enterobacter spp. (27), Proteus mirabilis (17), Citrobacter freundii (2), Providencia stuartii (3) and Serratia spp. (5), with various MICs of imipenem, were examined for production of metallo-beta-lactamases (MBLs) with different phenotypic laboratory tests that have been previously published to detect MBLs in Pseudomonas aeruginosa and Acinetobacter spp. Methods: A total of 194 (95 MBL-positive and 99 MBL-negative) clinical isolates with imipenem MICs <= 0.25 to > 256 mg/L were examined. All isolates were evaluated by the double-disc synergy test (DDST), the combination disc test (CDT), the MBL Etest and the modified Hodge test. The presence of bla(VIM) and bla(IMP) genes was evaluated by in situ hybridization with specific probes and was certified by PCR. Results: In 30 bla(VIM)-positive isolates that exhibited MICs of imipenem <= 4 mg/L, MBL Etest could not be evaluated. CDT with ceftazidime and 1900 or 750 mu g of EDTA, and DDST after applying an imipenem disc 10 mm apart from a disc containing similar to 1900 mu g of EDTA, showed the highest sensitivity (97.9% to 100%) and specificity (87.9% to 96%) rates among the analysed procedures. CDT with imipenem and 1900 mu g of EDTA exhibited a sensitivity of 94.7% and showed very good specificity (98%). Conclusions: The CDT with imipenem/imipenem+0.5 M EDTA or ceftazidime/ceftazidime+0.2 M EDTA and the DDST with imipenem 10 mm apart from EDTA are the most effective methods for the detection of MBLs in Enterobacteriaceae.