National-scale insights into AMR transmission along the wastewater-environment continuum.

The circulation of antimicrobial resistance (AMR) bacteria between human populations and the environment is a key driver of the global AMR burden, with wastewater acting as a major route of transmission. In this nationwide study, influent and effluent samples were collected from 47 municipal wastewater treatment plants (WWTPs) across Wales, covering areas of varying sociodemographics and representing approximately 66 % of the population connected to the main sewer network. Additionally, 76 river and estuarine sediment samples were collected upstream and downstream of the WWTPs, as well as from nearby recreational beaches. High-throughput qPCR was used to quantify 76 antimicrobial resistance genes (ARGs), 10 mobile genetic elements and 5 pathogens. Our analyses revealed that the absolute abundance and composition of the influent resistome was influenced by increasing WWTP catchment population size and density. Significant shifts in the resistome were observed following the wastewater treatment process, with the biological treatment stage identified as a critical determinant of AMR removal efficiency. WWTPs using biological filter beds were found to be more effective in reducing ARG relative abundances compared to those employing activated sludge processes. Despite the presence of ARGs in the effluent, the abundance and diversity of the river sediment resistomes did not increase downstream of the WWTPs. However, the presence of a resistome was found in all sediment samples, with varying compositions influenced by WWTP size and sediment source. Altogether, these findings highlight the complex and interconnected factors that shape the resistome across the wastewater-environment continuum, highlighting the need for comprehensive, nationwide surveillance studies to inform targeted interventions and mitigate the spread of AMR.