Background: Bloodstream infection (BSI) caused by carbapenem-resistant Enterobacteriaceae are potentially life-threatening related to poorer outcomes. Colistin is considered one of the last-resort treatments against human infections caused by multidrug-resistant (MDR) Gram-negative bacteria. Therefore, emergence of strains from the blood that co-harboring mcr  and carbapenem resistance genes were considered as a serious problem.
Purpose: In this study, two mcr-9 -harboring MDR Enterobacter cloacae  isolates BSI034 and BSI072 recovered from BSI patients were identified, one of which co-harbored mcr-9  and bla _NDM-1. The genetic characteristics of the MDR plasmid needed to be clarified.
Methods: S1-PFGE and Southern blotting were conducted to determine the location of mcr-9 . Whole-genome sequencing was performed to obtain the complete genome and plasmid sequences. The resistome and virulence genes of the strains, accompanied by the genetic characteristics of mcr-9 - and bla _NDM-1-harboring plasmids, were analyzed.
Results: Whole-genome sequencing showed that BSI034 harbored mcr-9 -carrying IncHI2-type pBSI034-MCR9 and bla _NDM-1-carrying IncX3-type pBSI034-NDM1. The 278,517 bp pBSI034-MCR9 carried mcr-9  along with the other 19 resistance genes. mcr-9  was flanked by IS 903B  (1057 bp) and IS 26  (820 bp) in the same orientation. In addition to resistance genes, strain BSI034 also carried a chromosome-located Yersinia  high-pathogenicity island, which harbored genes of yersiniabactin biosynthesis operon ybtSXQPAUTE, irp1/2 , and fyuA .
Conclusion: We described the complete genome and mcr-9 /bla _NDM-1-co-harboring plasmid of E. cloacae  from a BSI patient. Notable differences were observed within mosaic modules between pBSI034-MCR9 and other mcr-9 -harboring plasmids due to extensive recombination via horizontal gene transfer.
Keywords: MDR, mcr-9 , colistin, virulence genes