Emergence and diversity of IncHI2 plasmids co-carrying carbapenemase genes and mcr-9.1/9.2 in Enterobacter hormaechei from a Chinese children’s hospital

• All carbapenem-resistant E. hormaechei strains carry carbapenemase genes. • First report of bla NDM-1 and mcr-9.2 located on a single plasmid in China. • Conjugative IncHI2 plasmids exhibit high stability and transferability. • Genetic contexts of resistance genes involve diverse transposons and integrons. : This study aimed to characterize the IncHI2 plasmids co-harboring carbapenemase genes and mcr-9 genes in carbapenem-resistant Enterobacter hormaechei (CREH) isolated from a children's hospital in Suzhou, China. : Whole-genome sequencing (WGS) was employed to determine the genome sequences of 12 non-duplicate CREH strains. The molecular features of CREH strains and IncHI2 plasmids harboring carbapenemase genes with or without mcr-9 genes were characterized through genomic analysis. Conjugal transfer assay and plasmid stability testing were used to evaluate the transfer ability and stability of IncHI2 plasmids. : The genomic analysis revealed that 12 CREH strains all carried carbapenemase genes bla NDM-1 , bla NDM-5 or bla IMP-26 . Although the sequence types (STs) of these 12 strains were relatively diverse, most of the strains (11/12) carried IncHI2 and IncHI2A replicons. Further plasmid analysis showed that carbapenemase genes bla IMP-26 and bla NDM-1 could coexist with mcr-9.1 or mcr-9.2 on IncHI2 plasmids. While the backbones of these IncHI2 plasmids were relatively conserved, the accessory modules carrying the carbapenemase and mcr-9 genes exhibited diverse genetic contexts, involving transposon Tn 125 , Tn 1696 and Tn 6725 , and integron In27-like, In615b, and In837. Conjugal transfer assay and plasmid stability testing confirmed the transferability and stability of IncHI2 plasmids. : This study reports the emergence and diversity of IncHI2 plasmids that co-harbor carbapenemase genes and mcr-9.1/9.2 in E. hormaechei. To our knowledge, this is also the first report of bla NDM-1 and mcr-9.2 being carried on a single plasmid in China. The prevalence of such evolutionarily diverse and conjugative carbapenemase-encoding IncHI2 plasmids warrants close monitoring.