Clonal spread of mcr-1 in PMQR-carrying ST34 Salmonella isolates from animals in China

Since initial identification in China, the widespread geographical occurrence of plasmid-mediated colistin resistance gene mcr-1 in Enterobacteriaceae has been of great concern. In this study, a total of 22 Salmonella enterica were resistant to colistin, while only five isolates which belonged to ST34 Salmonella enterica serovar Typhimurium (S. Typhimurium) were mcr-1 positive. Four of them shared nearly identical PFGE type, although they were from different host species and diverse geographical locations. All the mcr-1-positive S. Typhimurium exhibited multi-resistant phenotypes including ampicillin, streptomycin, gentamicin, florfenicol, nalidixic acid, tetracycline, trimethoprim-sulfamethox, in addition to colistin. The oqxAB and aac(6')-Ib-cr genes were present alone or in combination in four (80.0%) and five (100%) isolates, respectively. The mcr-1 gene was located on a transferable IncI2 plasmid in the four genetically related strains. In the other one strain, mcr-1 was located on an approximately 190 kb IncHI2 plasmid. In conclusion, we report five mcr-1-positive S. Typhimurium/ST34 isolates. Both clonal expansion and horizontal transmission of IncI2-type plasmids were involved in the spread of the mcr-1 gene in Salmonella enterica from food-producing animals in China. There is a great need to monitor the potential dissemination of the mcr-1 gene.