Background: The emergence of the plasmid-borne colistin-resistant gene (mcr-1 ) poses a great threat to human health. What is worse, the recent observations of the coexistence of mcr-1  with carbapenemase encoding genes in some bacteria caused even more concern. Yet, there is a lack of observations of such strains in the human gut.
Methods: The isolation of E. coli  L889 was performed on selective medium plates. Antibiotic susceptibilities were determined by an agar dilution and a broth microdilution method. Multi-locus sequence typing (MLST) and acquired resistance genes were also characterized. Transferability of bla _NDM-9/mcr-1 -carrying plasmids was determined by conjugation, replicon typing and S1-Pulsed-field gel electrophoresis (S1-PFGE), and Southern blotting. The sequences of these plasmids were analyzed by using whole-genome sequencing with Illumina Novaseq and Nanopore platforms.
Results: E. coli  L889 was identified as ST1101 concomitantly carrying bla _NDM-9 and mcr-1  from a stool sample. Antimicrobial susceptibility tests showed that it was resistant to various antimicrobial agents and only susceptible to tigecycline. Notably, bla _NDM-9 was located on a ∼ 114-kb untypable plasmid, while mcr-1  was located on a ∼ 63-kb IncI2 plasmid.
Conclusion: Our research, to our knowledge, first reported an ST1101 E. coli  strain with an untypeable bla _NDM-9-harbouring plasmid and an IncI2 mcr-1 -carrying plasmid. The colonized E. coli  strains potentially contribute to the dissemination and transfer of bla _NDM-9 and mcr-1  to clinical isolates, which is a considerable threat to public health and should be closely monitored.
Keywords: bla _NDM-9, mcr-1 , gut, Escherichia coli , ST1101