Insights into Norfloxacin biodegradation and fate of antibiotics resistant genes (ARGs) associated with sulfur-driven autotrophic denitrification (SAD)

Norfloxacin (NOR) is one of the most consumed broad spectrum fluoroquinolone antibiotics. The presence of both nitrogen contaminants and NOR has been extensively reported so far in the low C/N wastewater. In this study, the sulfur-based autotrophic denitrification (SAD) associated NOR biodegradation system was established towards the simultaneous removal of nitrate and NOR. The routes of NOR abatement were investigated via a sequence of batch experiments. The findings illustrated that NOR removal was mainly contributed to the co-metabolic degradation with nitrate, while adsorption also played a role for NOR removal. The extracellular polymeric substance (EPS) content elevated under long-term exposure to NOR at 0.1 mg/L, which was conducive to resistance and adsorption of refractory contaminants. In addition, the NOR degradation pathways in the SAD system were identified based on the detection of the transformation intermediate and Fukui analysis. And the toxicity of the transformation by-products was assessed. The contents of genes coding cytochrome P450 (CYP450) enzyme, closely associated with NOR biodegradation, were markedly increased after long-term NOR exposure. Functional denitrifying genera of Thiobacillus exhibited great tolerance under the NOR pressure. The abundance of genes related to N, S metabolism was also influenced differentially by the presence of NOR. According to the metagene analysis results, antibiotic efflux and antibiotic target alteration were two important resistance mechanisms to NOR exposure in addition to the antibiotic inactivation, and the relative abundance of antibiotics resistant genes (ARGs) increased, which may be closely related to the horizontal and vertical gene transfer of ARGs. This study provides an incisive insight into NOR-like antibiotics biodegradation in SAD sludge system. We tried to open up an innovative opportunity for treating low C/N ratio wastewater towards the simultaneous removal of nitrogen pollutants and trace emerging contaminants as well as explore of the fate of ARGs.