Genomic characterization of conjugative plasmids carrying the mcr-1 gene in foodborne and clinical strains of Salmonella and Escherichia coli

ABSTRACT To date, the genomic characterization of foodborne bacteria carrying the mobile colistin resistance gene mcr-1 and transferability of mcr-1-carrying plasmids between wild-type bacteria have not been studied in detail. We retrospectively investigated the prevalence of mcr-1-positive Salmonella and Escherichia coli isolated from retail food products, food supply chain, and clinical samples across China in the last decade (2006–2017). The horizontal transfer of mcr-1-bearing plasmids and their complete nucleotide sequence were characterized using conjugation, S1-nuclease pulsed-field gel electrophoresis, and Southern hybridization, coupled with Illumina and Nanopore sequencing. Twelve (0.67%) mcr-1-positive strains were identified from 1,789 non-duplicated isolates, including 1,283 Salmonella and 506 E. coli. Eleven (21.7‰) mcr-1-positive E. coli strains were identified from the isolates collected during 2006–2017, which was significantly more prevalent than mcr-1-positive Salmonella in this period (0.78‰, 1/1,283; P