Environmental risks under individual and co-exposure to nisin and cetylpyridinium chloride: From denitrification system disturbance to resistance gene dissemination.

Nisin is a common natural food preservative, and cetylpyridinium chloride (CPC) is a typical quaternary ammonium compounds often applied as a disinfectant to food surfaces. Nisin and CPC frequently coexist in wastewater treatment plants and might pose adverse effects on the microorganisms responsible for nitrogen removal. This short-term batch experiment explored the responses of denitrification systems including nitrogen removal performance, cytotoxicity, resistance genes (RGs), and microbial community under individual and co-exposure to various concentrations of nisin and CPC. 5-80 mg/L nisin had little effect on the denitrification system, whereas 0.5-8 mg/L CPC individually or in combination with 5-80 mg/L nisin caused concentration dependent deterioration of denitrification performance, with total nitrogen removal efficiency even declining to below 5% at the highest tested concentrations. Furthermore, CPC individually or in combination with nisin reduced cell viability, altered microbial community structure, decreased the abundance of the denitrifying functional bacterium Thauera, and enriched Pseudomonas. Nisin and CPC might act synergistically, promoting the proliferation of RGs, with RGs free in water increasing to 9.90 times that of CK under co-exposure to 80 mg/L nisin and 8 mg/L CPC. Nisin and/or CPC might induce co-selection among different RGs, enhancing cross-resistance of bacteria. Positive correlations were observed between mobile genetic elements (intI1, tnpA-04) and multiple RGs, intensifying the potential risk of RGs spreading via horizontal gene transfer. This study revealed the environmental risks of nisin and CPC, providing a reference for assessing their impacts on the biological nitrogen removal process and the potential threat of RGs dissemination.