Comparison of Phenotypic Methods for Detection of Metallo-Beta- Lactamases in Gram Negative Bacteria at a Tertiary Care Hospital

Metallo-β-lactamase (MBL)-producing bacteria lead to resistance to carbapenem an antibiotic that is used as the last resort for treatment of multidrug-resistant bacteria, extended spectrum beta-lactamases, and AmpC β-lactamase-producing Gram-negative bacteria (GNB). The emergence of MBL-producing GNB is challenge to microbiology laboratories because there are no standardized guidelines available to detect them. The aim of this study was to compare three phenotypic methods to detect MBL production in GNB and to determine antibiotic sensitivity of MBL-producing isolates. A total of 250 GNB isolated from various clinical samples were tested for MBL production. All isolates were subjected to antibiotic susceptibility testing. Out of 250 isolates, 130 Imipenem (IPM)- resistant clinical isolates were taken as positive for MBL screening which were then subjected to confirmation by three phenotypic methods including Combined Disc test, MIC and Carba NP test. Out of 130 GNB, 112(86%) isolates were Carba NP positive for MBL production whereas the 18(13.8%) were negative for the enzyme. CDST and MIC, done for the presence of MBL enzyme revealed 104(80%) isolates to be positive for its presence whereas 26(20%) isolates were negative. In this study Carba NP test was found to be most sensitive and rapid method.