Comparative evaluation of phenotypic methods for detection of MBL producing Pseudomonas aeruginosa strains isolated from burn wound infections

Ntroduction: Pseudomonas aeruginosa is a notorious nosocomial pathogen with a resistant drug profile and a predominant isolate in burn wound infections. Recently Metallo beta lactamase (MBL) producing isolates have emerged particularly in P.aeruginosa leading to failure of therapy with carbapenems with increased mortality and morbidity in burn units. Objectives: To detect MBL production among the imipenem resistant isolates of P.aeruginosa by different phenotypic methods – Modified Hodge test (MHT), Combined disc test (CDT), Imipenem-EDTA Double disc synergy test (DDST) and MBL-E-test. Materials and Methods: A total of 108 clinical isolates of P.aeruginosa obtained in 189 pus samples collected from different burn wound sites, were subjected to antimicrobial susceptibility testing by modified Kirby-Bauer disc diffusion method. All the imipenem resistant isolates were further tested for MBL-production by MHT, CDT, DDST and MBL-E-test. Results: Out of 108 isolates of P.aeruginosa 26 (24.07%) ware imipenem resistant. Among 26 imipenem resistant isolates 16(61.54%) were detected as MBL producers by MBL-E-test, 15(57.69%) by DDST, 14(53.85%) by CDT and 11(42.31%) by MHT. Considering MBL-E-test as standard, maximum sensitivity was shown with DDST (93.75%) followed by CDT (87.5%) and MHT (68.75%). Conclusion: A comparative evaluation of MHT, CDT and DDST against MBL-E-test proved DDST to be most effective, with higher sensitivity and specificity. DDST is observed to be an economical feasible alternative compared to MBL-E-test as a regular screening test in MBL detection in critically ill.