Clonal outbreak of NDM-1-producing Enterobacter hormaechei belonging to high-risk international clone ST78 with the coexistence of tmexCD2-toprJ2 and mcr-9 in China

Objectives: The emergence of carbapenem-resistant Enterobacter cloacae complex (CR-ECC) has posed sig-nificant global challenges to the clinical treatment of healthcare-associated infections. This study reports the clonal outbreak of NDM-1-producing Enterobacter hormaechei ( E. hormaechei) with the coexistence of tmexCD2-toprJ2 and mcr-9 in China.Methods: During the outbreak (January 2018 - December 2021), 15 non-repetitive multidrug-resistant E. hormaechei strains were obtained from 13 patients in a tertiary hospital. Antimicrobial susceptibil-ity testing, plasmid stability, plasmid conjugation, plasmid fitness evaluation, colistin induction, whole-genome sequencing, and bioinformatics analysis were performed. A phylogenetic tree was constructed based on single nucleotide polymorphisms of core genomes to illustrate the evolutionary dynamics of mcr-9-carrying E. hormaechei strains worldwide. Results: The 15 E. hormaechei strains belonged to the high-risk international clone ST78 and co-harboured tmexCD2-toprJ2 and blaNDM-1, of which 12 E. hormaechei strains carried the mcr-9 gene. Whole-genome se-quencing analysis revealed that tmexCD2-toprJ2 and blaNDM-1 coexisted on the IncFIB/IncFII-type plasmid, which could be transferred to Escherichia coli J53 by conjugation and had a significant effect on host fitness. The mcr-9 gene was located between two insertion sequences, IS903B and IS1R, but lacked the two-component system regulatory gene qseBC, which might be the reason for all mcr-9-positive E. hor-maechei strains remaining susceptible to colistin. The expression of mcr-9 was not inducible in strains confirmed by colistin induction assays. Phylogenetic analysis illustrated the silent spread and rapid evo-lution of mcr-9-carrying E. hormaechei worldwide.Conclusion: This study enriched the epidemiological and genomic characterisation of the coexistence of tmexCD2-toprJ2 and mcr-9 in ST78 CR-ECC isolates and demonstrated that they could prolong clonal dis-semination in a tertiary hospital in China. Continuous epidemiological surveillance and molecular char-acterisation of CR-ECC should be conducted to monitor the evolution of CR-ECC around the world.(c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

This study reports the outbreak of NDM-1-producing Enterobacter hormaechei in a tertiary hospital in China. The strains belonged to a high-risk clone and co-harbored tmexCD2-toprJ2 and blaNDM-1, with 12 strains also carrying mcr-9. The study highlights the rapid evolution and potential for clonal dissemination of these strains, emphasizing the need for enhanced surveillance and control measures.