Hydrogeochemical drivers of antimicrobial resistance (AMR) in the Maipo river basin, Chile: the role of treated wastewater effluent

Antimicrobial resistance remains an imminent and serious public health challenge. Wastewater presents an important reservoir for antimicrobial resistance evolution and transmission in the environment. Here, we investigated antimicrobial resistance and water quality markers in the Maipo River basin, Chile, in a ∼120 km longitudinal study of predominantly surface water (n = 13) in Santiago Metropolitan Region. Pollution indicators (e.g. NO3−, dissolved organic carbon (DOC), tryptophan-like fluorescence, IS1247, 16S rRNA, merA and crAss64) increased from upstream to downstream in the Mapocho and Maipo rivers and particularly downstream from Santiago and/or following wastewater discharge. There was no detectable AMR in sampled groundwater (n = 2; 50–120 m) nearby to the wastewater treatment plant, plausibly because the sampled wells represent relatively evolved groundwater with longer residence times, compared to river water. Wastewater tracers (Tryp-like fluorescence, log(16S rRNA), DOC, crAss64, PO4) indicated strong associations with IS1247 (p < 0.01), whilst DOC, PO4, and log(16S rRNA) were associated with sul1 (p = < 0.05). Results suggest mobile genetic elements (MGEs) such as insertion sequences may be promoted by, or enriched in, treated wastewater. These findings improve our understanding of how treated wastewater affects downstream water quality in populated river basins.