Background: Colistin acts as the last line of defense against severe infections caused by carbapenem-resistant Enterobacteriaceae . Infections caused by extensively drug-resistant isolates coproducing MCR and carbapenemases have posed a serious public health concern.
Purpose: In this study, we reported the first clinical colistin and carbapenem-resistant Enterobacter hormaechei  isolate SCNJ07 coharboring bla _NDM-1 and mcr-9  from a patient with bloodstream infection in China.
Methods: Bacterial antimicrobial susceptibility testing was performed using the broth microdilution method. Conjugation assay was carried out to investigate the transferability of mcr-9  and bla _NDM-1. Whole-genome sequencing of strain SCNJ07 was performed using an Illumina HiSeq system and the genetic characteristics of the mcr-9 - and bla _NDM-1-harboring plasmids were analyzed.
Results: Conjugation assays revealed that both bla _NDM-1 and mcr-9  genes could successfully transfer their resistance phenotype to Escherichia coli  strain J53. Whole genome sequencing showed that SCNJ07 possessed an FIB36:FIIY4 type self-transmissible plasmid bearing bla _NDM-1, which possessed high similarity to previously reported pRJF866 in China. mcr-9  was located on a ∼28-kb self-transmissible plasmid pMCR-SCNJ07 with both IncHI2 and IncR replicons. Two copies of intact IS903 that bracketed a ∼8-kb region containing the mcr-9  gene were identified in pMCR-SCNJ07. BLASTn analysis revealed that a number of mcr-9 -positive plasmids have been around for a while among Enterobacteriaceae  worldwide.
Conclusion: This study reveals the likelihood of a wide dissemination of this newly identified colistin resistance gene mcr-9  among Enterobacteriaceae . Further surveillance is urgently needed to understand the prevalence and dissemination of mcr-9 , thereby facilitating establishment of measures to control its spread.
Keywords: colistin, mcr-9 , CRE, IS903