Human and livestock fecal source tracking for antibiotic resistance genes in rural private well water

Authors: STOKDYK, JOEL - Us Geological Survey (USGS); Opelt, Sarah; FIRNSTAHL, AARON - Us Geological Survey (USGS); Cook, Rachel; Heffron, Joseph; MULDOON, MAUREEN - Wisconsin Geological And Natural History Survey; Burch, Tucker

Submitted to: American Society for Microbiology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 2/28/2024
Publication Date: 6/15/2024
Citation: Stokdyk, J.P., Opelt, S.A., Firnstahl, A.D., Cook, R.M., Heffron, J.A., Muldoon, M., Burch, T.R. 2024. Human and livestock fecal source tracking for antibiotic resistance genes in rural private well water. American Society for Microbiology Annual Meeting. June 13-17, 2024.

Interpretive Summary:

Technical Abstract: Antimicrobial resistance poses a significant public health problem, with 700,000 deaths from drug-resistant infections per year globally. Groundwater is a reservoir of antibiotic resistance genes (ARGs), which originate in human wastewater and livestock manure. Individual ARG sources, like livestock farms and municipal landfills, have been linked to ARG contamination of water. However, the relative contribution of livestock and human feces to ARG contamination of groundwater is unknown because few studies consider multiple fecal sources simultaneously. We examined ARG occurrence in water from private wells (n = 138) in rural southwest Wisconsin, USA, where groundwater is influenced by livestock manure and wastewater from private septic systems. Water was sampled using both large-volume ultrafiltration (mean sample volume = 853 L) and one-liter grab samples in four seasonal events (34 or 35 wells per event). Samples were concentrated and tested by quantitative polymerase chain reaction for 11 ARGs, the class 1 integron gene (intI1), and 17 microbial source tracking markers for human and livestock feces. Private wells were contaminated by human wastewater (64 wells), cow manure (33 wells), and pig manure (13 wells), including 23 wells contaminated by multiple fecal sources. Of 138 wells, 117 were positive for at least one ARG or intI1. Nine of 11 ARGs were detected rarely, in 14 (10%) or fewer wells each. The targets sul1 (72% wells), tetW (33% wells), and intI1 (16% wells) were detected in more wells. Sixty-five percent of ARG detections co-occurred with human wastewater or livestock manure as determined by microbial source tracking. The likelihood of detecting individual ARGs differed by fecal source, with detection of some ARGs more likely when human wastewater was present than livestock manure and vice versa. Across all ARGs, more detections were associated with wastewater than manure, consistent with the greater number of wastewater-positive wells. The ARG detections that occurred in the absence of human, cow, or pig feces (35% of total ARG detections) may indicate background occurrence of these genes in groundwater or analytical limitations (fecal sources not tested or false-negative source tracking results). Overall, ARG contamination of private wells was common in the rural study area, and both human and livestock feces were associated with ARG detections. Examining ARGs where multiple fecal sources influence wells allows a novel assessment of the relative contributions of manure and septic systems to ARG occurrence in groundwater.