Introduction: The emergence of acquired metallo-β-lactamases (MBL) in Gram-negative bacilli is becoming a therapeutic challenge, as these enzymes usually possess a broad hydrolysis profile that includes carbapenems, extended-spectrum β-lactams. Aim: To detect Extended spectrum β-lactamases and metallo-β-lactamase in carbapenem resistant Gram negative clinical isolates from various clinical specimens and to evaluate their antibiotic susceptibility patterns. Material and Methods: A total of 100 non duplicates imipenem resistant isolates were tested for the presence of extended spectrum β-lactamases by phenotypic confirmatory test, metallo-β-lactamases by Double disk synergy test with various distances from edge to edge (10mm,15mm,20mm), between the IPM and EDTA and combined disc test. Result: Of the 100 IMP resistant isolates screened 30 (30%) were MBL positive by phenotypic methods, i.e., double disk synergy test and combined disc test. Co-existence of Extended spectrum β- lactamases and MBL were detected in 3 (30%). All the 30 MBL positive isolates had shown synergy at (100%) at 10 mm distance, 27 (90%) isolates had shown synergy at 15 mm distance and 13 (43.4%) isolates were shown synergy at 20 mm distance. All the 30 MBLs producers were multidrug resistant and 27 (90%) were sensitive to colistin (CL). All MBL positive Pseudomonas aeruginosa were sensitive to polymyxin B (100μg). Conclusion: Microbiologists are now facing a challenge of drug resistance due to MBL production. Although CLSI guidelines do not quote about the ESBL detection in Pseudomonas aeruginosa MBLs and ESBL have to be detected in them. The use of combination tests would increase the sensitivity to detect the presence of MBL among the clinical isolates of Gram-negative bacilli. The spread of MBL producing Gram negative organism can be prevented if they are detected in all isolates and routinely adopted in all laboratories.
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