Evaluation of three phenotypic methods for the detection of metallo-beta-lactamase production in non fermenting gram negative bacilli

Abstract

Background and objectives: Non-fermenting gram-negative bacilli (NFGNB) are common nosocomial pathogens and are often resistant to several antibiotics in clinical practice. Metallo-beta-lactamase (MBL) mediated resistance to carbapenems is an emerging threat among hospital isolates of P.aeruginosa. As there are no CLSI guidelines for the detection of these enzymes the present study was conducted with the objective to evaluate three phenotypic methods for the detection of Carbapenemase and MBL production in NFGNB. Methods: 100 consecutive NFGNB resistant to ceftazidime and imepenem by disc diffusion test were included in the study. Resistance to imepenem was confirmed by determining the minimum inhibitory concentration (MIC) by agar dilution method. Three phenotypic tests, namely modified Hodge test (MHT), ethylene di-amine tetra acetic acid (EDTA) imepenem double disk synergy test (DDST) and 4-fold reduction in MIC with imipenem-EDTA combination were evaluated for detection of carbapenemase and MBL production. Results: Of the 100 multidrug resistant NFGNB isolated 66 were P. aeruginosa and 34 were Acinetobacter species. 62 P.aeruginosa and 31 Acinetobacter isolates were confirmed resistant to imepenem by MIC, 7 isolates had MIC values in the intermediate range. 55/100 imepenem resistant isolates were MBL producers by MHT. 73/100 isolates were positive for MBL production by DDST. 74/100 isolates showed more than fourfold fall in MIC with IMP-EDTA combination. In this study 74% of imepenem resistant NFGNB were MBL producers. Interpretation and Conclusion: P.aeruginosa was the predominant NFGNB isolated. DDST and MIC with imipenem EDTA combination are almost equally effective and more sensitive than MHT for detection MBL production. Routine screening for MBL mediated carbapenem resistance among clinical isolates of NFGNB is important to ensure appropriate treatment of infections caused by them in clinical settings.