Plastisphere as a resistome incubator: Substrate biodegradability escalates compounded genetic risks

Plastispheres formed on microplastics represent unique hotspots for antibiotic-resistant bacteria, yet how substrate biodegradability shapes resistome profiles remains unclear. To address this gap, this study evaluated the resistome and its associated mobility and pathogenicity risks across nine polymer-specific plastispheres and the surrounding water. Results showed that antibiotic resistance gene (ARG) enrichment was widespread across plastispheres formed on all tested substrates, following a biodegradability-driven gradient that peaked in polyhydroxyalkanoate (PHA). This enrichment was systematically coupled with the co-accumulation of mobile genetic elements (MGEs) and virulence factor genes (VFGs). Host-resolved analysis revealed the selective enrichment of high-risk biomarker taxa in biodegradable plastispheres, particularly Aeromonas and Escherichia, which concurrently harbored antibiotic resistance, genetic mobility, and virulence determinants. Genetic-context analysis revealed contig-level co-localization of these determinants, indicating their potential genetic linkage. Within this structural framework, the co-occurrence of ARGs and MGEs indicated an increased potential for MGE-mediated horizontal gene transfer, whereas the enrichment of VFGs involved in environmental sensing, colonization, and nutrient acquisition suggested potential adaptation to resource competition, thereby potentially contributing to the co-selection of genetically linked ARGs. Substrate biodegradability amplifies both pathways by intensifying these spatial and competitive interactions, rendering plastispheres critical incubators for resistant pathogens in riverine ecosystems. Ultimately, this resistome expansion independent of antibiotic selective pressure reveals the potential ecological hazards of biodegradable plastics, substantiating the need for a systematic reassessment of their widespread use within the "One Health" framework.