Persistent mcr-1.1 E. coli in poultry despite Japan’s colistin ban
Colistin is a polymyxin antibiotic used as a last-resort treatment for severe infections caused by multidrug-resistant Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. Due to its importance in human medicine, colistin is classified by the WHO as a critically important antimicrobial. However, its widespread use in veterinary settings - particularly in livestock for disease prevention and growth promotion - has raised concerns about emerging resistance. Of particular concern is the plasmid-mediated colistin resistance gene mcr-1, first reported in China in 2016, which facilitates horizontal transfer among bacterial populations, thereby threatening the efficacy of this essential antibiotic.
A study published in Antibiotics investigated the prevalence and characteristics of colistin-resistant E. coli strains isolated in 2024 from broiler chickens across five poultry farms in Japan. Among 147 E. coli isolates, 20 (13.6%) exhibited resistance to colistin. Whole-genome sequencing revealed that all resistant strains harbored the mcr-1.1 gene on an IncI2 plasmid - a mobile genetic element capable of horizontal gene transfer. Multilocus sequence typing identified all isolates as ST1485, indicating clonal dissemination across farms. Core-genome comparisons showed that these strains were highly similar locally but genetically distinct from global ST1485 isolates.
Despite the Japanese government's 2018 ban on colistin as a feed additive, colistin resistance persists in broiler farms, suggesting the gene remains endemic.
