Evaluation of modified carbapenem inactivation method with CARBA NP, MHT and real-time PCR for detection of carbapenemase-producing Enterobacteriaceae

Abstract

Background: Carbapenemase-producing Enterobacteriaceae (CPE) are a global emerging threat with high transmissibility, multidrug-resistance and limited treatment options. Reliable detection is important for prevention, containment of these pathogens as well as for antimicrobial stewardship. The modified carbapenem inactivation method (mCIM) is an inexpensive and straightforward phenotypic tool for detecting carbapenemase activity. The aim of this study was to evaluate the performance of mCIM for detecting carbapenemase activity by comparing with CARBA NP, Modified Hodge Test (MHT) and Real-time PCR. Methods and materials: A total of 94 carbapenem-nonsusceptible Enterobacteriaceae comprising of E. coli (n = 43), Klebsiella species (n = 33) and Enterobacter species (n = 18) were analyzed by mCIM, MHT and CARBA NP according to the Clinical and Standard Institute (CLSI) guidelines (M100-S27). In-house Real-time PCR was performed to confirm carbapenemase genes (KPC, NDM, VIM, IMP, OXA-48). The sensitivity and specificity of mCIM, MHT and CARBA NP was determined by comparing the results with Real-time PCR. Klebsiella pneumoniae BAA-1706 strain and Klebsiella pneumoniae ATCC 700603 both carbapenemase non-producers were used as negative controls. Results: Of the ninety-four isolates analyzed by real-time PCR, 47 isolates expressed NDM and 5 isolates expressed OXA. 38 isolates showed co-expression of NDM and OXA. Two showed co-expression of KPC and NDM. One Enterobacter aerogenes showed co-expression of NDM and VIM, while another showed co-expression of NDM and IMP. The sensitivity of mCIM, CARBA NP and MHT were 100%, 84% and 74% respectively. mCIM showed higher sensitivity than CARBA NP & MHT. The mCIM assay perfectly correlated with the genotypic findings of the 94 isolates with 100% sensitivity and specificity for CPE detection. Conclusion: The results of our study show that mCIM is suitable and reliable for detecting carbapenemase-producing Enterobacteriaceae than CARBA NP and MHT. Compared to molecular method, mCIM is less expensive and can be readily adopted in clinical microbiology laboratory.