Response of activated sludge to antibiotics stress: Neglected legacy effects

The microbial communities of activated sludge could be affected by antibiotics pollution. However, it is unclear that how the historical antibiotic stress feedback to the later responses of microbes and antibiotic resistance genes (ARGs) to combined antibiotics stress by leaving their imprint on activated sludge (“legacy effect”). In order to conceptualize the role of antibiotic legacy effects, sulfamethoxazole (SMX) or trimethoprim (TMP) was selected as historical stress to investigate responses of activated sludge to combined SMX and TMP stress (0.005–30 mg/L). Ammonia removal was affected by legacy of activated sludge delivered, which increased with the increase of historical antibiotic concentrations, but it was less affected by the legacy of antibiotic type stress. Changes of aromatic protein-like substances and soluble microbial by-products in soluble microbial product (SMP) and tightly bound extracellular polymeric substances (TB-EPS) were more obvious under combined SMX and TMP stress, which could reflect the dynamic response of microorganisms to antibiotics stress. Legacy effects affected response of structure and composition of microbial community to later antibiotic stress. ARGs could produce positive feedback on the increase of relative abundance of hosts under antibiotics stress to occupy the niche of bacteria with high relative abundance, especially genera (Taibaiella, Hydrogenophaga, et al.) with low proportion but carrying multiple ARGs (mdtD, mdtG, acrD). Overall, legacy of activated sludge delivered is able to change its responses to antibiotics stress, and the occurrence of ARGs can drive enrichment of their potential hosts.