mcr-Mediated Colistin Resistance in Escherichia coli: A One Health Review of Reported Prevalence Across Human, Animal, Food, and Environmental Sectors.

Antimicrobial resistance (AMR) is a growing global health problem that threatens the effective treatment of bacterial infections. Escherichia coli is widely used as an indicator organism for monitoring AMR because it circulates among humans, animals, food, and the environment. As resistance to many commonly used antibiotics has increased, colistin has been reintroduced as a last-resort treatment for severe infections caused by multidrug-resistant Gram-negative bacteria. The discovery of plasmid-mediated colistin resistance genes (mcr) has raised serious concern, as these genes can spread easily between bacteria. This narrative review synthesizes findings from 28 selected studies published up to 2025, covering more than 36,000 E. coli isolates collected from human, animal, food, and environmental sources across different regions of the world. mcr-positive E. coli were most commonly reported in livestock and meat products, with moderate occurrences in environmental samples and generally lower prevalence in human clinical isolates, although direct comparisons are limited by substantial study heterogeneity. These findings suggest that food-producing animals and contaminated environments represent important reservoirs of colistin resistance, whereas human exposure may occur through indirect pathways, although prevalence data alone cannot establish transmission directionality. Even with restrictions on colistin use in animal production and improved antimicrobial stewardship in many countries, mcr genes persist due to factors like environmental contamination and co-selection with other resistance genes. This persistence is influenced by environmental contamination, the presence of other resistance genes on the same plasmids, and global trade of food products. The available evidence underscores that colistin resistance cannot be effectively addressed within a single sector. A coordinated One Health approach that integrates human health, animal production, and environmental management is essential to limit the further spread of mcr-mediated colistin resistance and to preserve the clinical value of this critical antibiotic.