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 spread antimicrobial resistance (AMR). This study compared for the first time systematically livestock and human waste streams with respect to the abundance and environmental release of QACs, QAC resistance indicator genes (qacE/qacEΔ1), and QAC-tolerant bacteria. Five livestock farms with on-farm biogas plants (BGPs) as well as a rural and an urban municipal wastewater treatment plant (WWTP) were studied in parallel. In WWTPs, less than 1% of incoming QACs were discharged with treated wastewater but 10-20% were transferred to sewage sludge. QAC concentrations in dewatered and digested sewage sludge (biosolids) far exceeded those in raw and digested manure. Relative abundances of QAC-tolerant heterotrophic bacteria cultivated under nutrient rich conditions at 37°C (growth conditions for potential human pathogens) 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 elevated QAC tolerance were cultivated from human waste streams. Most QAC-tolerant strains also carried antibiotic resistances without strong system differences. While qacE/qacEΔ1 genes were detected in all samples, only few cultivated BAC-C12-tolerant strains carried these genes, indicating that other mechanisms must be responsible for increased QAC tolerance. In conclusion, QACs, qacE/qacEΔ1, and QAC-tolerant heterotrophic bacteria cultivated under incubation conditions optimal for faecal derived facultative pathogenic bacteria were detected in both livestock and human waste streams. Human waste streams appear particularly relevant for their release into the environment.