Microplastics exacerbate antibiotic resistance by regulating microbial and functional gene dynamics in sludge and food waste composting.

The presence of antibiotic resistance genes (ARGs) and microplastics (MPs) as co-contaminants in sewage sludge possess environmental concern. Nevertheless, the effect of specific MP types on ARGs propagation and the mechanistic drivers during composting require further elucidation. This study analyzed the impact of MPs polyethylene (PE), polypropylene (PP), and their mixture (PE+PP) on the propagation of ARGs and mobile genetic elements (MGEs) during sludge composting. Results revealed that MPs significantly increased ARGs abundance, with the highest enrichment observed in PE (2.06 log-fold), followed by PP (1.74) and PE+PP (1.65), compared to control trial. MGEs also increased by up to 1.3 log-fold in MP-treated groups. MPs altered bacterial community composition, enhancing the abundance of Brevibacterium, Microbacterium, and Streptomyces. MPs exhibited a partial reduction of up to 30.2 % during composting which modified the compost's chemical properties, elevating carbon content and depleting oxygen. Metagenomic analysis revealed MPs affected microbial functional pathways, upregulating ABC transporters, two-component regulatory systems, and the conjugation gene VirD4. Functionally, MPs enhanced denitrification and dissimilatory nitrate reduction to ammonium, inhibited assimilatory nitrate reduction, and promoted inorganic nitrogen assimilation. Redundancy analysis exhibited MGEs as the primary drivers of ARGs proliferation in MP-amended compost, while temperature was dominant in the control. Network analysis identified key bacterial hosts Staphylococcus, Weissella, Lactiplantibacillus, Sphingobacterium carrying multiple ARGs and MGEs. This study highlights the potential of MPs, even at low concentrations, to promote ARG dissemination by altering microbial communities and enhancing gene transfer, offering new insights into ecological risks associated with MP-contaminated composting systems.