Tracking environmental resistance through surveillance of ESBL- and colistin-resistant Escherichia coli in livestock waste for One Health action.

Antimicrobial-resistant bacteria are emerging biological contaminants that threaten both ecosystems and human health. Antimicrobial resistance (AMR) in livestock waste is increasingly recognized as an environmental hazard of global concern, with implications for food safety and public health. This study examined the occurrence and environmental behavior of extended-spectrum β-lactamase (ESBL)-producting and colistin-resistant Escherichia coli under environmentally relevant conditions in livestock waste from pig, cow, goat, chicken, and duck farms in Thailand. Cefotaxime-resistant E. coli were detected in all animal types, with pig, cow, and duck farms serving as major reservoirs. Untreated pig wastewater contained the highest levels of resistant E. coli (70% prevalence; 5.13-7.55 log10 CFU/100 mL), while treatment reduced but did not eliminate contamination. Among 78 confirmed isolates, all were resistant to ampicillin and cefotaxime, 5.1% to ceftazidime, and none to carbapenems. Phenotypic colistin resistance occurred in 6.4% of isolates. All isolates carried blaCTX-M genes, mainly blaCTX-M group 1 (85.9%), while blaCTX-M group 9 (15.4%) was also detected. Plasmid-mediated mcr genes (mcr-1 to mcr-3) were identified in 5.1% of isolates, exclusively from untreated pig wastewater, confirming these effluents as localized hotspots for resistance dissemination. Multiple antibiotic resistance indices ranged 0.29-0.43, with the highest values in pig farm isolates, indicating moderate to high levels of multidrug resistance. Overall, livestock wastewater represents a significant source of microbial contaminants. This study provides field-based evidence to support risk assessment and mitigation of antimicrobial-resistant bacteria within the One Health framework, emphasizing the importance of improving waste management to protect environmental and public health.