Microplastics host distinct microbial communities enriched in potentially pathogenic taxa and antibiotic resistance genes: evidence from environmental and experimentally aged plastics in a coastal marine environment

Microplastics (MPs) represent persistent pollutants in marine environments and may act as substrates for complex microbial biofilms, collectively referred to as the plastisphere. However, the extent to which microplastic-associated communities differ from surrounding water microbiota and contribute to the environmental dissemination of potentially pathogenic taxa and antibiotic resistance genes (ARGs) remains incompletely understood. In this study, we investigated environmental MPs collected in the Northern Adriatic Sea together with polyethylene MPs experimentally aged for eight months under mesocosm conditions. Using 16S rRNA gene amplicon sequencing, shotgun metagenomics and droplet digital PCR, we compared MP-associated biofilms with the corresponding marine and lagoon water microbiota and assessed selected ARG profiles. Microplastic-associated biofilms displayed significantly lower alpha-diversity and distinct beta-diversity patterns compared with free-living communities, indicating that MPs harboured microbial communities differing from the surrounding water microbiota. Both environmental and experimentally aged MPs showed a higher relative presence of potentially pathogenic genera, with Vibrio representing the dominant genus within the potentially pathogenic component of the plastic microbiota. Shotgun metagenomic analysis revealed a richer and more diverse resistome on environmental MPs compared to seawater, including resistance classes exclusively detected on MPs. Digital droplet PCR confirmed higher normalized abundances of key ARGs ( tetA , tetB , sul2 ) on MPs, together with the preferential detection of clinically relevant genes such as mcr-1 and qnr variants. By integrating environmental MPs, experimentally aged polyethylene MPs, 16S rRNA gene amplicon sequencing, shotgun metagenomics and ddPCR validation, this study provides a combined environmental and experimental assessment of MP-associated microbial communities and selected ARGs in marine and lagoon environments. Together, these findings demonstrate that marine MPs may act as persistent biologically active substrates for biofilm-forming microbial communities that differ from surrounding free-living communities and may contribute to the spread of ARGs, highlighting the need for integrated monitoring of MPs pollution and antimicrobial resistance.