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

•The inhibitory effect of QACs on denitrification ranked: BAC > ATMAC > DADMAC.•Inhibition of QACs on denitrification might relate to the reduction of Thauera.•QACs caused a significant decrease in ROS and CCK and inhibited EPS secretion.•QRGs abundance in both sludge and water all increased under shock loads of QACs.•High concentration BAC boosted greatly in wi-MGEs, wi-ARGs, we-ARGs and we-QRGs.