Beyond Antimicrobial Activity: Plant-Derived Bioactive Compounds as Modulators of Antimicrobial Resistance in Broiler Chickens—Mechanisms, Resistome Dynamics, and One Health Implications

Antimicrobial resistance (AMR) in broiler production is a One Health concern that extends beyond the occurrence of resistant pathogens to the persistence, mobility, and dissemination of antimicrobial resistance genes (ARGs) within intestinal and environmental microbial communities. Plant-derived bioactive compounds are increasingly proposed as alternatives or complements to conventional antimicrobials; however, antibacterial activity against resistant bacteria does not necessarily demonstrate AMR mitigation. This critical narrative review evaluates phytochemicals as resistance-modifying agents in broiler chickens, distinguishing direct antibacterial effects from antibiotic potentiation, resistance-mechanism interference, ARG and mobile genetic element (MGE) modulation, horizontal gene transfer inhibition, and community-level resistome effects. Studies involving selected plant-derived compounds and phytogenic preparations have reported effects on antimicrobial susceptibility, efflux-associated activity, resistance enzymes, plasmid transfer, ARG transcript abundance, and resistome composition across different bacterial species and experimental models. However, these findings are compound-, model-, and endpoint-specific, and direct evidence from broiler chickens remains limited and heterogeneous. Outcomes are strongly dependent on phytochemical composition, dose, formulation, intestinal exposure, bird age, microbial ecology, and analytical methodology, while repeated subinhibitory exposure may under some conditions promote tolerance or antibiotic cross-resistance. From a One Health perspective, reductions in intestinal ARG abundance or resistant bacterial populations should therefore be considered initial rather than definitive evidence of AMR mitigation. Future progress will require chemically standardized interventions evaluated through integrated phenotypic, genomic, transcriptomic, and resistance-mobility approaches. Longitudinal studies across the poultry production continuum are also needed to determine whether these interventions produce durable reductions in resistance burden and dissemination.