In Vitro Efficacy of Meropenem-Cefmetazole Combination Therapy against New Delhi Metallo-β-lactamase-producing Enterobacteriaceae

Abstract Background Limited treatment options complicate management of infections with New Delhi metallo-β-lactamase (NDM)-producing organisms. We assessed the efficacy of combination therapy with meropenem (MEM) and cefmetazole (CMZ) against NDM-producing Enterobacteriaceae. Materials and Methods : Twelve E. coli clinical isolates harbouring blaNDM-1 along with a positive control E. coli BAA-2469 harbouring blaNDM-1 were studied. Minimum inhibitory concentrations (MICs) of MEM, ertapenem (ERT), and CMZ were determined by broth microdilution. Checkerboard and time-kill assays were performed to confirm the in vitro efficacy of the MEM/CMZ combination. Scanning electron microscopy, kinetic studies, and whole genome sequence analysis were used to determine the antimicrobial resistant mechanisms. Results : MICs of MEM, ERT, and CMZ in monotherapy ranged from 8 to 32, 16 to 128, and 32 to 512 μg/mL, respectively. In checkerboard, MEM/ERT resulted in no synergy, whereas MEM/CMZ showed a synergistic effect in all the tested isolates. Furthermore, the MIC of MEM in combination decreased by 2- to 8-fold compared to that of MEM alone. The time-kill study revealed a bactericidal effect in 4 out of 13 isolates at 24 hours. Scanning electron microscopy showed spheroidisation of the bacterial cell in the MEM/CMZ combination, which was not observed in single antibiotic conditions. The kinetic studies indicated a better activity of CMZ to ERT as an antagonist for NDM-1. Whole genome sequence analysis did not reveal any explainable differences between isolates susceptible to the combination therapy and those not susceptible to combination therapy. Conclusion : In vitro studies showed the potential effectiveness of MEM/CMZ combination therapy against NDM-producing organisms.