Abstract
Untreated combined sewage (bypass) is often discharged by wastewater treatment plants to receiving rivers during stormwater events, where it may contribute to increased levels of antibiotic resistance genes (ARGs) and multi-resistance risk factors (multi-resistant bacteria and multi-resistance genomic determinants (MGDs)) in the receiving water. Other contamination sources, such as soil runoff and resuspended river sediment could also play a role during stormwater events. Here we report on stormwater event-based sampling campaigns to determine temporal dynamics of ARGs and multi-resistance risk factors in bypass, treated effluent, and the receiving river, as well as complimentary data on catchment soils and surface sediments. Both indicator ARGs (qPCR) and resistome (ARG profiles revealed by metagenomics) indicated bypass as the main contributor to the increased levels of ARGs in the river during stormwater events. Furthermore, we showed for the first time that the risk of exposure to bypass-borne multi-resistance risk factors increase under stormwater events and that many of these MGDs were plasmid associated and thus potentially mobile. In addition, elevated resistance risk factors persisted for some time (up to 22 h) in the receiving water after stormwater events, likely due to inputs from distributed overflows in the catchment. This indicates temporal dynamics should be considered when interpreting the risks of exposure to resistance from event-based contamination. We propose that reducing bypass from wastewater treatment plants may be an important intervention option for reducing dissemination of antibiotic resistance.
Highlights
Antibiotic resistance has been considered one of the biggest chal lenges to public health, and its global increase has been recognized as an impending public health crisis by intergovernmental entities
In this study we investigated the effect of stormwater events leading to bypass discharge on a highly wastewater-impacted river, the River Murg near the WWTP Münchwilen, Switzerland
This site was well studied under dry-flow condi tion in 2018 (Lee et al, 2021), and has the following characteristics: (1) Wastewater effluents comprises a high proportion of the downstream river discharge (33.0–38.0% under base-flow condition), (2) Under baseflow conditions, no other known point-source of wastewater inputs exist other than studied WWTP, (3) All potential combined sewer overflows into the river are from the sewer system that connects to the studied WWTP
Summary
Antibiotic resistance has been considered one of the biggest chal lenges to public health, and its global increase has been recognized as an impending public health crisis by intergovernmental entities. Antibiotic resistance genes (ARGs) and bacteria are transmitted via direct human-to-human interaction, and via multi-sectoral routes, such as interdependent routes among humans, food animals, and environments (McEwen and Collignon, 2018). ARGs and bacteria are enriched in reservoirs such as human patients, hospitals, and livestock, sometimes driven by antibiotic overuse (McEwen and Collignon, 2018). ARGs and bacteria can subsequently be transmitted to environments via sewage, hospital wastewaters, farm runoff, manure, etc. The levels of common ARGs and bacteria in treated effluents are significantly higher than upstream wa ters by up to 2 order of magnitude (Lee et al, 2021; Rodriguez-Mozaz et al, 2015; Sabri et al, 2018). The levels of ARGs and bacteria at receiving waters increase after receiving effluents (Lee et al, 2021; Rodriguez-Mozaz et al, 2015), even though the exact degree of increase depends on the proportion of effluent discharge to river discharge (Lee et al, 2021; Ort and Siegrist, 2009)
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