Comparison of two methods to detect carbapenemase & metallo-β-lactamase production in clinical isolates

Background & objectives: Bacterial resistance has greatly hampered effective treatment of patients in clinical settings. Non-fermenting Gram-negative bacilli (NFGNB) are common nosocomial pathogens. In this study we attempted to develop a convenient test for early detection of carbapenemase and metallo-β-lactamase (MβL) production in NFGNB. Lack of sufficient reports from India in this area indicated the need for this study. Methods: A total of 50 imipenem resistant NFGNB were speciated, and their resistance reconfirmed by disk diffusion and minimum inhibitory concentration (MIC) determination by agar dilution. Two different methods namely modified Hodge and EDTA disk synergy tests were evaluated for carbapenemase and metallo-β-lactamase (MβL) production. Results: Of the 50 imipenem resistant NFGNB, 48 and two respectively fell in the resistant and intermediate range in MIC using agar dilution. Majority of these were Pseudomonas aeruginosa (n=28), followed by Burkholderia cepacia (n=9). The modified Hodge test could detect 28 strains as carbapenemase and MβL producers, while the EDTA disk synergy test was able to detect an additional 8 strains producing MβL and carbapenemase. Interpretation & conclusion: Pseudomonas aeruginosa was found to be the predominant NFGNB in our hospital setting and EDTA disk synergy could detect more carbapenemase and metallo-β-lactamase producers compared to modified Hodge test.