Objective: A colistin-resistant Escherichia coli  strain isolated from dog feces was characterized in this study.
Methods and Results: A multiplex PCR assay was used to detect the presence of colistin-resistant mcr  genes; it was found that E. coli  QDFD216 co-harbored the mcr-1  and mcr-3  genes. Whole-genome sequencing and further bioinformatics analysis revealed that E. coli  QDFD216 belonged to serotype O176:H11, fimH1311 type and ST132. The resistance genes bla _CTX-M-14, mdfA, dfrA3, acrA, acrB, tolc , and sul3  were present in the chromosome. The mcr-1.1  and mcr-3.7  genes were located in two plasmids of different incompatibility groups. mcr-1.1  was carried by a IncX4-type plasmid within an typical IS 26-parA-mcr-1.1-pap2  cassette, while mcr-3.7  was encoded by an IncP1-type plasmid with a genetic structure of TnAs2-mcr-3.7-dgkA -IS 26 . No additional antibiotic resistance genes were carried by either plasmid.
Conclusion: This is the first report of an E. coli  isolate co-harboring a mcr-1.1 -carrying IncX4 plasmid and a mcr-3.7 -carrying IncP1 plasmid. The evolution and mechanism of mcr  gene co-existence need further study to assess its impact on public health.
Keywords: colistin resistance, whole-genome sequencing, mcr  genes, mcr-1 , mcr-3