Extracellular Polymeric Substances Govern Horizontal Gene Transfer of Antibiotic Resistance Genes: Mechanisms, Pathway Specificity, and Research Gaps

The dissemination of antibiotic resistance genes (ARGs) in environmental systems is recognized as being shaped by local matrix conditions, yet the pathway-specific role of extracellular polymeric substances (EPS), the dominant matrix of biofilms, sludge flocs, and granules, remains poorly resolved. In this Review, we develop a mechanism- and pathway-resolved perspective for evaluating how EPS govern horizontal gene transfer (HGT) of ARGs. Current evidence indicates that conjugation is the most directly supported pathway, with EPS facilitating or restricting transfer through effects on donor–recipient contact, matrix organization, and transfer-related physiology. For transformation, EPS-associated extracellular ARGs constitute reservoirs, but EPS–DNA binding can both preserve genetic material and limit its immediate bioavailability. By contrast, EPS involvement in phage-mediated transduction and vesicle-mediated transfer remains mechanistically plausible but insufficiently validated. We identify five interconnected mechanisms: physical retention and spatial organization, DNA adsorption and bioavailability, regulation of intercellular contact, modulation of cell physiology and transfer competence, and community-scale mobilome coupling. By clarifying evidence boundaries across pathways, this Review moves EPS–HGT research from correlation-based interpretation toward mechanism-informed assessment of ARG mobility. We conclude by outlining research needs, including standardized EPS definition and extraction, component-resolved characterization, pathway-resolved causal assays, and validation in realistic environmental aggregates.