LAUSR

Background The aim of this study was to clarify the molecular characterization of NDM-1-producing carbapenem-resistant Enterobacter cloacae complex (CREL) at a teaching hospital in Chongqing, China. Methods Antimicrobial susceptibility and resistance genes were analyzed. Epidemiological relationship was analyzed by pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). Conjugation experiments were performed to determine the transferability of plasmids. Whole-genome sequencing (WGS) of strains was implemented, and the genetic environment of the bla NDM-1- and mcr-9-carrying plasmids was analyzed. Results A total of 10 bla NDM-1-positive CREL isolates were identified. All isolates harbored multiple resistance genes. ECL68 and ECL78 co-produce bla NDM-1 and mcr-9. Among the four different sequence types (STs) detected, ST1466 was assigned as a novel ST. Six isolates exhibited highly similar PFGE patterns. Conjugation assay proved that all plasmids containing bla NDM-1 or mcr-9 could be transferred to the recipient Escherichia coli. WGS indicated that bla NDM-1 genes were carried by diverse plasmids, including IncHI2/IncN, IncX3, and one unclassified plasmid type. The backbone structure of these plasmids is involved in replication initiation (repAB), partitioning (parABM), and conjugation/type IV secretion (tra/virB). Analysis of the genetic environment showed that bla NDM-1 in three plasmids exhibited a highly similar structure to protype Tn125. Co-existence of bla NDM-1 and the colistin resistance gene mcr-9 was detected in the two isolates, ECL68 and ECL78. In ECL68, bla NDM-1 and mcr-9 were present on the same plasmid while located in two separate plasmids in ECL78. The genetic environment of mcr-9 was organized as IS26-wbuC-mcr-9-IS903-pcoS-pcoE-rcnA-rcnR, and the two-component system encoding genes qseC and qseB was not found in two plasmids, which could explain mcr-9-harboring strains’ colistin susceptibility. Conclusions We first report a nosocomial outbreak of NDM-1-producing E. cloacae complex ST177 in China. Conjugative plasmids contributed to the horizontal transfer of antibiotic resistance genes. The prevalence and even coexistence of bla NDM-1 and mcr-9 may further threaten public health. Our results highlight further surveillance for bla NDM-1, and mcr-9 is essential to prevent its dissemination.