Colistin residues and colistin-resistant Enterobacteriaceae in agricultural soils: Sources, risks, and remediation strategies

Polymyxins, including colistin, are critical last-line antibiotics, and their environmental dissemination raises One Health concerns. This review synthesizes current evidence on the occurrence, sources, environmental fate, and mitigation of colistin residues and colistin-resistant Enterobacteriaceae in agricultural soils, with emphasis on transmission pathways to crops and implications for food safety along the farm-to-fork continuum. Principal inputs from livestock manure, reclaimed wastewater, and wildlife are characterized. Resistance mechanisms, with a focus on plasmid-mediated mobile colistin resistance (mcr), are summarized. Although animal manure may be a significant source of colistin due to its low gastrointestinal absorption, soil concentrations are low, with bioavailability influenced by physicochemical parameters, including pH, clay content, cation exchange capacity, and organic matter content. Low desorption rates limit plant uptake; thus, the primary environmental risk arises from the selection and enrichment of colistin-resistant bacteria and mcr genes in the rhizosphere, as well as splash-mediated deposition of contaminated particles. In farm and arable soils, mcr-1 and mcr-3 have been identified as the dominant variants, with higher prevalence in livestock-associated environments. Their dissemination is primarily driven by horizontal gene transfer rather than clonal expansion, influenced by factors such as soil characteristics, heavy metals, soil treatments, and plant root exudates. Interventions are critically appraised, spanning veterinary stewardship and on-farm hygiene, physical processes, chemical approaches, and biological strategies, along with postharvest barriers that include Good Agricultural Practices and Hazard Analysis and Critical Control Points, washing and sanitization, and bacteriophage biocontrol. Major conclusions are that multi-barrier, context-specific programs can reduce environmental selective pressures and interrupt gene flow while maintaining agronomic viability, yet progress remains constrained by gaps in standardized surveillance (particularly for plant-based foods), and by the limited use of quantitative risk assessment and field-scale validation of remediation technologies. A One Health framework that integrates environmental monitoring with public-health endpoints is needed to guide proportionate policy and practice.