Long-term spatiotemporal dynamics of β-lactamase resistome in the Han River, South Korea.

The global dissemination of antimicrobial resistance, particularly resistance to widely prescribed β-lactam antibiotics, is a critical public health threat worldwide. Aquatic ecosystems act as key reservoirs where antibiotic resistance genes accumulate and are transmitted between environmental bacteria and human pathogens within a One Health framework. To move beyond short-term observations and better understand these cross-sectoral dynamics, we conducted 5-year longitudinal surveillance using shotgun metagenomics across different seasons along the Han River, South Korea. Our analysis revealed a diverse repertoire of β-lactamases, predominantly belonging to Ambler classes A and D, which also accounted for most mobile β-lactamase genes and were enriched in densely populated downstream regions. Furthermore, β-lactamase genes exhibited clear seasonal fluctuations, with a pronounced winter surge in total and mobile β-lactamase gene abundances. We also identified a substantial fraction of novel variants, whose abundance and composition varied across spatial and seasonal gradients. Network analysis revealed that the genera