Reducing Antimicrobial Resistance in Poultry Carcasses Extends Beyond Farm-Level Interventions.

The aim of this study was to assess how raising chickens without the use of antimicrobials affects the microbiome of poultry carcasses. A total of 151 caeca and neck skin samples from chickens raised without antimicrobials were collected in the same slaughterhouse and submitted to shotgun metagenomic sequencing. Caeca were dominated by Bacillota and Bacteroidota, while carcasses by Pseudomonadota. The caeca microbiome was enriched in genes related to a proliferating and metabolically active microbial community. Carcass-associated microbiomes were enriched in functional genes linked to adaptation to nutritionally limited and oxidative environments. A significantly higher cumulative antimicrobial resistance gene abundance was detected in carcasses compared to caeca. Specifically, carcasses exhibited approximately 1.5 times more AMR genes, reflecting an increase of nearly 49%. While caeca showed enrichment of resistance determinants associated with Gram-positive anaerobic gut commensals, carcasses were characterized by a predominance of multidrug efflux systems and clinically relevant β-lactam resistance genes, commonly associated with environmental and opportunistic Gram-negative bacteria. In carcasses, carbapenem-associated genes, such as OXA-58-like and CphA, were detected. However, these genes have not been associated with carbapenemase-producing Enterobacterales. Overall, the findings of this study indicate that reducing antimicrobial resistance in food animal production systems extends beyond farm-level intervention. At present, the benefits of the interventions aimed at reducing antimicrobial resistance at farm level seem to be compromised during the post-harvest stages.