Long-term effects of oxytetracycline (OTC) on the granule-based anammox: Process performance and occurrence of antibiotic resistance genes

The effect of oxytetracycline (OTC) on anaerobic ammonium oxidation (anammox) performance and the evolution of anammox consortia under OTC stress were analyzed in this study. Meanwhile, the level of resistance genes under OTC stress was monitored for the first time in the anammox system. The nitrogen removal capacity of anammox reactor was almost deprived within three weeks under the stress of 2 mg L−1 OTC, accompanied by a significant reduction of specific anammox activity of 81.3% and heme c content of 50.1%. However, the abundance of hydrazine synthase gene was adversely affected, with a loss of 29.7%. The efflux pump antibiotic resistance genes tetA, tetB, and tetC and the enzymatic modification gene tetX were first detected in anammox mixed culture. The abundance of tetA was increased from 1.03 to 2.51%. The ratios of tetB and tetC doubled, while there was no obvious change in the ratios of tetX. Therefore, it can be inferred that anammox microorganisms resisted the biotoxicity of OTC via efflux pumping mechanism. After the withdrawal of OTC from the influent, the nitrogen removal capacity of anammox reactor recovered, and the abundance of resistance genes declined gradually.