Identification of a novel hybrid plasmid coproducing MCR-1 and MCR-3 variant from an Escherichia coli strain

Objectives: To characterize the genome of an Escherichia coli harbouring both mcr-1 and mcr-3.19 on a hybrid plasmid and the underlying transmission mechanisms. 

Methods: Broth microdilution was used to perform antimicrobial susceptibility testing. Conjugation assays and S1-PFGE were used to assess the transferability of mcr genes. Resistance genotypes and genetic contexts were investigated, based on WGS data from the Illumina and MinION platforms. Inverse PCR was performed to test the mcr-3.19-bearing circular intermediate. Bioinformatic tools were used to further characterize the hybrid plasmid. 

Results: E. coli CP53 was identified as harbouring both mcr-1 and mcr-3.19 on a 231 859 bp hybrid plasmid pCP53-mcr1-3 containing IncFIA, IncHI1A, IncHI1B and IncN replicons. The genetic structures of mcr-1 and mcr-3.19 were similar to those reported in other mcr-1 and mcr-3.19-bearing plasmids, which suggested that recombination between mcr-bearing plasmids had been mediated by ISs. However, the MDR plasmid pCP53-mcr1-3 cannot transfer via conjugation. Furthermore, another three plasmids were identified in the isolate, two of which encoded resistance genes. In640 duplication between two MDR plasmids was observed. An MDR-region recombination existed in E. coli CP53. A core structure consisting of mcr-3-dgkA existed in mcr-3-bearing plasmids reported, to date. Circular intermediates were observed for mcr-1 and mcr-3.19 regions. 

Conclusions: A novel mcr-3.19 was identified along with mcr-1 contained in a hybrid plasmid. This finding suggested that evolution of mcr genes among various plasmids was being driven by mobile elements. Molecular surveillance of mcr gene co-occurrence warrants further investigation to evaluate the public health risk.