Microbiome and resistome dynamics in different stages of commercial broiler production with restricted antimicrobial use

BACKGROUND: Antimicrobial use (AMU) in poultry production is central to curb the Antimicrobial Resistance (AMR) crisis. Institutional and market pressure led many commercial poultry operations to practice distinct levels of AMU restriction. On-farm data remains one of the main bottlenecks in understanding the impacts of AMU restriction at the farm level and across production systems. However, AMR dynamics in company-wide production chains remain largely unexplored, precluding improvement of AMU policies and stewardship. STUDY AIM: Here, we shotgun sequenced soil and litter samples from 26 poultry farms and carcass rinses from a processing plant to reconstruct the microbiome and resistome of two vertically integrated commercial poultry operations to explore their dynamics under AMU restriction. RESULTS: Shotgun sequencing revealed that litter microbiome and resistome changed significantly by production stage and company, reflecting management practices and possible effects of historical AMU. Meanwhile, broiler farms had increased detection of potential pathogens and AMR diversity. We found no evidence of farm-to-fork transmission. Effective biosecurity protocols largely maintained the separation between the internal and external environments of the poultry houses, except on two farms where breaches might have led to external spread of pathogens and AMR. CONCLUSION: Our study highlights that AMR in commercial poultry system reflects the combined effect of production-stage, company practices, and environmental boundaries. Future studies should integrate quantitative AMR data and culture-based techniques with metagenomic findings to strengthen tracking and surveillance of AMR in poultry farm environments.