Detection of Metallo β Lactamase in lactose non-fermenting gram negative organisms-comparison of different methods

Background & Objectives: Increasing incidence of bacterial resistance to β lactam antibiotics is a potential healthcare hazard. In majority of the cases, this resistance is orchestrated through production of β lactamases. Among these, Carbapenemases, especially transfer able metallo β lactamases (MBL), are the most important as they hydrolyze many antibiotics. MBL genes are often plasmid mediated and hence have potential for rapid dissemination. This study was conducted to phenotypically detect MBL in lactose non-fermenting Gram negative bacilli and to compare the different methods. Methods: Strains of Pseudomonas aeruginosa and Acinetobacter spp. isolated from different clinical specimens were included in the study. MBL was detected using EDTA-Imipenem (EIC) & EDTA-Ceftazidime (ECC) combination assay and EDTA-Imipenem (EDTA-IPM) & EDTA-Ceftazidime (EDTA-CAZ) double disc synergy test (DDST). Carbapenemase was detected using Modified Hodge test (MHT). Results: A total of 54 strains of P.aeruginosa and 55 strains of Acinetobacter spp. were studied. MBL was detected in most number of strains of P.aeruginosa using the EDTA-IPM-DDST (75%) method whereas the EIC and the ECC methods detected MBL in most number (53% each) ofstrains of Acinetobacter spp. MHT could detect Carbapenemase in 22% and 40% strains of P.aeruginosa and Acinetobacter spp. respectively. Interpretation &Conclusions: Our results suggest that while EDTA-IPM-DDST is better method for MBL detection in P.aeruginosa, the EIC and ECC assays are equally good for MBL detection in Acinetobacter spp.MHT can only be used for screening of Carbapenemases in Acinetobacter spp.