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The decrease in transformation frequency sunlight-induced dissolved organic matter inhibits eARG transformation in microplastic biofilms at a single-cell and community-wide level
Summary
Microplastics floating in water often host bacterial films that can spread antibiotic resistance genes—but this study found some good news: natural organic matter in water, when exposed to sunlight, creates reactive molecules that damage bacteria and disrupt the process by which they pick up resistance genes from their environment. This suggests that sunlight and natural water chemistry may act as a built-in brake on antibiotic resistance spreading through microplastic pollution, which is relevant since these resistant bacteria can eventually make their way into water supplies, seafood, and ecosystems humans depend on.
Microplastic (MP) biofilms serve as hotspots for the dissemination of extracellular antibiotic resistance genes (eARGs) via natural transformation. Dissolved organic matter (DOM) is a ubiquitous photosensitizer in aquatic systems, yet its net effect on eARG transformation within MP biofilms remains unclear. This study shows that environmentally relevant DOM under simulated sunlight inhibited eARG transformation in MP biofilms at the single-species level. DOM (5-40 mg/L) reduced natural transformation frequency to 0.55-0.87 times under simulated sunlight. DOM photochemistry also markedly decreased bacterial density and extracellular polymeric substance (EPS) components in MP biofilms. Mechanistic investigations using quenching experiments demonstrated that reactive oxygen species (ROS) photochemically generated from DOM played a dominant role in mediating this inhibitory effect. DOM photochemistry downregulated the expression of transformation-related functional genes, thereby impairing cellular competence for eARGs uptake and integration. A consistent inhibitory trend was observed in a multi-species system derived from natural water communities. The transformants identified in MP biofilms belonged to 4 phyla and 48 genera, with Escherichia (phylum Proteobacteria) exhibiting absolute dominance (95.22%-98.47%). The decline in eARG transformation frequency in MP biofilms was mainly attributable to a reduction in transformant abundance. Neutral community modeling further suggested that DOM under light imposed substantial deterministic selection on transformant community assembly in MP biofilms. Collectively, these findings demonstrate that DOM photochemistry acts as an important environmental constraint on natural transformation-based eARG dissemination in sunlit MP biofilms.