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 colistinresistant 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 IS26-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-IS26. 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 coexistence need further study to assess its impact on public health.

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Background: The emergence and prevalence of plasmid-mediated colistin-resistant bacterial strains in recent years have raised great concerns in clinical medicine. It is urgently needed to develop a cheaper, faster, simpler, sensitive, and specific molecular detection method to identify and monitor the dissemination of the transferable resistant determinants. Methods and Results: Herein, eight pairs of primers were designed to set up a multiplex PCR method for the rapid and efficient determination of reported mcr genes. This assay can give results within 85 min (35 min for amplification and 50 min for electrophoresis). We validated the feasibility of this assay by testing the presence of mcr genes in 60 colistinresistant isolates. Conclusion: Our multiplex PCR technique exhibits remarkable advantages in the light of clear identification, efficiency of amplification, as well as the time consuming for detection, and thus could be useful for the surveillance and epidemiological research of plasmidmediated colistin resistance, particularly for the under-resourced laboratories.

Colistin was considered as the last-resort drug against severe clinical infections caused by multidrug-resistant Gram-negative pathogens. Mobile colistin resistance ( mcr ) genes and its variants carried by plasmids have been reported in diverse niches in recent years, and yet few studies reported carriage of mcr-10 in ECC strains of companion animal origin.