Metagenomic characterization of bacterial communities on beach macroplastics: Insights into antimicrobial resistance and virulence.

Macroplastic debris in coastal environments provides stable substrates for microbial colonization, yet comparative assessments with natural substrates remain limited. This study investigated bacterial communities associated with beach macroplastics collected from four sites along the Kochi coast, Kerala, India (Fort Kochi, Cherai, Puthenthode, and Puthuvypin) during the pre-monsoon season, and compared them with those colonizing natural inanimate substrates (driftwood, seaweed, and shells). Composite sampling across multiple transects was employed, and shotgun metagenomic sequencing was used to characterize taxonomic composition, functional pathways, antimicrobial resistance genes (ARGs), and virulence factors. Across all samples, Pseudomonadota (average ∼64.8%) dominated, followed by Bacillota, Actinomycetota, and Bacteroidota. Plastic-associated communities showed greater dominance of specific genera, including Vibrio, Alteromonas, and Pseudoalteromonas, whereas natural substrates exhibited more evenly distributed taxa (Streptomyces, Marinobacter, Sulfitobacter etc). Functional annotation revealed the presence of core metabolic pathways across all samples, while xenobiotic degradation and lipid metabolism pathways were more prominently represented in plastic-associated communities, particularly at urban-influenced sites. A total of 42 ARGs belonging to eight antibiotic classes were identified, with β-lactam resistance genes constituting ∼42% of detected ARGs. Plastic-associated samples showed broader ARG profiles, including blaTEM-116, tetM, and sul1. A total of 73 virulence genes were identified, with plastic samples showing higher abundance of β-lactamase (blaTEM-116, tetM) and adhesion-associated genes (pilA, ompA). In addition, 1264-2046 virulence-related gene hits per site were detected, with consistently higher counts observed in plastic-associated communities. Overall, the findings demonstrate that macroplastics support distinct microbial assemblages and functional gene distributions compared to natural substrates, highlighting their role as microbial habitats in human-impacted coastal environments.