Underappreciated role of polluted storm sewer discharge in the accumulation of antibiotic resistance genes in downstream sediments.

Polluted storm sewers (PSSs) are important pathways for contaminant transport to receiving waters, yet the association between PSS discharge and antimicrobial resistance in downstream sediments remains poorly understood. We investigated antibiotic resistance gene (ARG) distributions in paired upstream-downstream stream water and sediments around six PSS outfalls, together with PSS outfall samples, using metagenomic sequencing and binning. Downstream sediments exhibited transitional ARG profiles between upstream and PSS sediments, indicating a compositional linkage with PSS sediments. Acquired ARGs (sulfonamide and tetracycline resistance genes) were enriched in downstream sediments, while the intrinsic chromosomal resistance determinant mexB showed a similar pattern, consistent with their higher abundances in PSS discharges. SourceTracker indicated that sediment sources (87%) from upstream and PSSs contributed more to downstream sediment ARG profiles than water sources. The intermediate suspended solid levels in downstream water between PSS water and upstream water further supported particle-associated transport. Sulfonamide resistance genes were associated with putative plasmid contigs and co-occurred with qacEdelta1 and IS91, suggesting potential mobility. Azonexus was the dominant potential host of sulfonamide and tetracycline resistance genes, whereas Pseudomonas_E was associated with mexB enrichment. Collectively, these findings highlighted PSS discharge as an important anthropogenic pathway associated with ARGs in downstream sediments.