The role of gut microbiome in antimicrobial resistance transmission between companion animals and livestock: mechanisms, drivers, and One Health implications.

Antimicrobial resistance (AMR) poses a critical global public health challenge, with animal gut microbiomes serving as significant reservoirs and transmission hubs for antimicrobial resistance genes (ARGs). This review synthesizes current knowledge on the central role of gut microbiomes in companion animals and livestock in facilitating AMR dissemination. It examines key mechanisms that enable horizontal gene transfer within intestinal ecosystems: conjugation, transduction, and transformation. It also highlights how co-selection by heavy metals, disinfectants, and other non-antibiotic agents sustains resistance even without direct antibiotic use. The review analyzes major drivers of AMR, including antimicrobial usage, husbandry practices, and environmental pressures. It critically evaluates microbiome-based interventions such as probiotics, postbiotics, and fecal microbiota transplantation. A distinctive contribution is the integration of these elements into a network-centric One Health framework that explicitly maps cross-species transmission pathways from livestock and companion animals to humans via direct contact, food chains, and environmental dissemination. By moving beyond descriptive cataloging to provide a mechanistic and ecological synthesis, this review aims to guide the development of targeted, microbiome-informed intervention and surveillance strategies.