Co-selection may explain the unexpectedly high prevalence of plasmid-mediated colistin resistance gene in a Chinese broiler farm.

The rise of the plasmid-encoded colistin resistance gene is a major concern globally. Here, during a routine surveillance, an unexpectedly high prevalence of with reduced susceptibility to colistin (69.9%) was observed in a Chinese broiler farm. Fifty-three (63.9%) isolates were positive for . All identified -positive (MCREC) were multidrug resistant and carried other clinically significant resistance genes. Furthermore, the genes were mainly located on the IncI2 and IncHI2 plasmids. Conjugation experiments unraveled the co-transfer of with other antibiotic resistance genes (, , , and ) via the IncI2 (=3) and IncHI2 (=4) plasmids. The stable genetic context - was common in the IncI2 plasmids, whereas IS---IS was mainly found in the IncHI2 plasmids. The dominance of -bearing IncI2 and IncHI2 plasmids and co-selection of with other antimicrobial resistance genes might contribute to the exceptionally high prevalence of in this broiler farm. Our results emphasized the importance of appropriate antibiotic use in animal production.