Coexistence of bla-IncN and mcr-1-IncX4 plasmids in a ST48 Escherichia coli strain in China.

OBJECTIVES

The aim of this study was to explore the genomic content of a bla- and mcr-1-harbouring Escherichia coli strain and to clarify the molecular mechanism for the transmission of multidrug resistance.

METHODS

Antimicrobial susceptibility testing was conducted by the broth microdilution method to determine the resistance profile. Filter-mating conjugation assays were performed to confirm the plasmid transfer ability. Whole-genome sequence data were acquired by a combination of Illumina paired-end reads and Nanopore long-read sequencing.

RESULTS

Escherichia coli strain QE11-421 was an mcr-1-positive colistin-resistant isolate that co-harboured the bla gene conferring carbapenem resistance. Genome sequence data confirmed QE11-421 as a sequence type 48 (ST48) E. coli that harboured five large conjugative plasmids encoding several multidrug resistance genes. The bla gene was located on a Tn3-Tn4401 composite transposon, which is part of a 65-kb multidrug-resistant IncN plasmid. The mcr-1 gene was harboured on another 33-kb IncX4 plasmid that was more conserved and presented few antimicrobial resistance determinants. No copies of insertion sequence ISApl1 were found flanking the mcr-1 gene, decreasing the mobility of mcr-1 based on its original Tn6330 transposon.

CONCLUSIONS

Horizontal transfer of multidrug resistance plasmids or resistance-related transposon units was responsible for the emergence of this notorious superbug. The coexistence of bla-IncN and mcr-1-IncX4 plasmids in a ST48 E. coli strain in humans poses a great threat.