mcr-1 remains detectable in various Escherichia coli lineages isolated from healthy swine after withdrawal of colistin use on the farm

We monitored swine-derived Escherichia coli on a Japanese farm where colistin had been used for the treatment of diseases caused by bacteria and investigated colistin resistance and the presence of mcr-1 in 36 E. coli strains isolated before and after the withdrawal of colistin use. Through the withdrawal of colistin use on the farm, the prevalence of colistin-resistant and mcr-1-positive E. coli was markedly reduced but not eradicated because mcr-1 had been maintained in multiple plasmids and various sequence types of nonpathogenic E. coli carried in healthy swine. The monitoring of sequence types of mcr-1-positive E. coli is expected to be important for controlling colistin resistance in swine or other animals.