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 Pseudomonas and Klebsiella acted as primary hosts, with their mobile β-lactamase genes highly connected to multiple taxa, suggesting important roles in β-lactamase gene transmission. These findings suggest that preemptive management of aquatic β-lactamase genes is a fundamental strategy for mitigating future resistance crises within a One Health framework and provide critical evidence for identifying transmission pathways and tracking resistance evolution between natural and clinical environments.