Decreased colistin resistance and mcr-1 prevalence in pig-derived Escherichia coli in Japan after banning colistin as a feed additive

Background: Antimicrobial resistance to colistin, a widely used feed additive for farm animals across the world, has raised public health concern in recent years. Since July 2018, its use as feed additive has been banned in Japan to reduce the spread of plasmid-based mobilized colistin resistance (mcr) genes and the subsequent development of colistin-resistant bacteria. Evaluating the effects of these measures is required. Methods: We evaluated the effects of colistin use, as a feed additive, on colistin resistance in pigs (n = 5) from birth to finishing in the farm. Moreover, to evaluate changes in colistin resistance and mcr gene prevalence in response to colistin withdrawal, E. coli samples derived from pig faeces sourced from the fields of three geographically distinct farms were characterized before and after the withdrawal of colistin as a feed additive. Results: Colistin-resistant Escherichia coli in pigs (n = 5) increased during the colistin administration period and decreased immediately after its end. In three fields, the colistin resistance rate and prevalence of mcr-1 decreased immediately and significantly after the ban. However, colistin-resistant and mcr-1-positive E. coli were still detected in all three farm fields 12 months after the ban on colistin use. Conclusion: Agricultural colistin use caused selective pressure that contributed to widespread mcr dissemination in Japan. Colistin resistance and the presence of mcr genes should be continuously monitored in food-producing animals