Short-term stress of quaternary ammonium compounds on intracellular and extracellular resistance genes in denitrification systems

Abstract

The wastewater containing high quaternary ammonium compounds (QACs) concentrations from hospitals and other sources may cause the transient shock loads on the wastewater treatment plants (WWTPs). Denitrification is the key part of nitrogen removal from wastewater, however, the effects of QACs on the denitrification and the spread patterns of intracellular and extracellular resistance genes were still unknown. The short-term stress of three frequently detected QACs on denitrification were systematically investigated in this study. The inhibitory effect on nitrate removal had the following rules: BAC C12 > ATMAC C12 > DADMAC C12. The inhibition of QACs on denitrification performance was correlated with the reduction of Thauera abundance, and a higher QACs concentration made stronger inhibitory effect. Moreover, high concentrations of QACs could cause a significant decrease in reactive oxygen species, inhibit cell activity and extracellular polymeric substances secretion. Furthermore, QACs stimulated an overall increase of intracellular and extracellular QACs resistance genes in denitrification systems, both in sludge and in water. And co-selection among resistance genes occurred under shock loads of QACs, especially a high BAC C12 concentration caused a substantial raise in abundances of resistance genes in water. It was worth noting that resistance genes in sludge and water were mainly contributed by different bacteria. Rhizobium and Pseudomonas may be the potential hosts contributed to resistance genes in sludge while Thauera mainly contributed to that in water.