Quaternary ammonium compounds (QACs), QAC resistance genes, and QAC tolerant bacteria in livestock and human waste streams

Quaternary ammonium compounds (QACs) are high production volume biocidal compounds increasingly scrutinized for their potential to promote antimicrobial resistance spread. This study compared the release of QACs, QAC resistance indicator genes (qacE/qacE{Delta}1), and QAC tolerant bacteria from livestock and human waste streams into the environment. Five livestock farms with on-farm biogas plants (BGPs), a rural and an urban municipal wastewater treatment plant (WWTP) were studied in parallel. In WWTPs, <1% of incoming QACs were discharged with treated wastewater but 10-20% were transferred to sewage sludge. QAC concentrations in sewage sludge far exceeded those in raw and digested manure. The qacE/qacE{Delta}1 genes were detected in all samples with a higher relative abundance in solid than liquid samples. Relative abundances of QAC tolerant fast growing heterotrophic bacteria cultivated under high nutrient conditions at 37{degrees}C were higher in human than livestock waste streams. Providencia and Pseudomonas dominated the cultivated QAC tolerant bacteria in both systems but showed higher QAC tolerance when originating from human waste streams. Additionally, Enterobacteriaceae with higher QAC tolerance were cultivated from human waste streams. Most QAC tolerant strains carried antibiotic resistances without strong system differences. Only few strains carried the qacE/qacE{Delta}1 gene indicating that other mechanisms must be responsible for the increased QAC tolerance. In conclusion, QACs, qacE/qacE{Delta}1, and viable QAC tolerant bacteria including potential pathogenic bacteria were released from livestock and human waste streams into the environment with highest abundances in a post-pandemic sewage sludge sample. Highlights- QACs most abundant in human waste streams, especially biosolids - Higher relative abundance of QAC tolerant bacteria in human waste streams - Pseudomonas and Providencia dominated QAC tolerant bacteria in both waste streams - Enterobacteriaceae with higher QAC tolerance abundant in human waste streams - Most QAC tolerant strains carried additional antibiotic resistances Environmental implicationMunicipal wastewater treatment plants (WWTPs) and livestock farms are hotspots for antimicrobial resistance (AMR) propagation. We compared the simultaneous occurrence of quaternary ammonium compounds (QACs), resistance genes (RGs), QAC-tolerant bacteria, and their multidrug-resistance status in livestock and human waste streams. QACs, indicators of QAC tolerance and AMR occurred in both systems but were higher in WWTPs, especially sewage sludge. Our findings highlight the need for prudent disinfectant use and enhanced waste treatments to reduce the risks of spreading micropollutants, pathogens, and AMR via organic fertilizers or treated wastewater recycled in circular agricultural practice.