Tetracycline exposure shifted microbial communities and enriched antibiotic resistance genes in the aerobic granular sludge.

The aerobic granular sludge with larger size and more compact spherical structure generally shows excellent performance in antibiotic removal, yet little is known about the long-term effect of environmentally-relevant concentration (μg/L) of antibiotics on the proliferation of antibiotic resistance genes (ARGs) and microbial community in aerobic granules. Herein, a sequencing batch reactor (SBR) was set up with dosing different concentrations (0-500 μg/L) of tetracycline to investigate its influences on microbial communities and ARG levels in aerobic granular sludge. Results show that the bioreactor could effectively remove chemical oxygen demand (COD), nitrogen, and tetracycline during the long-term operation. The quantitative polymerase chain reaction (qPCR) analysis shows that tetracycline at μg/L level could greatly enhance the absolute and relative abundances of tetA, sulII, and bla in the effluent and aerobic granules, indicating tetracycline could serve as a selection pressure on the development of ARGs corresponding to different types of antibiotics in aerobic granules. Pearson's correlation analysis also implies that sulII and bla were correlated strongly with tetA. Moreover, the presence of tetracycline altered the microbial communities and diversity of the effluent and aerobic granules in the bioreactor. These findings would advance our understanding of the proliferation and development of ARGs in aerobic granules under tetracycline pressure and serve as a foundation to guide the application of aerobic granular sludge for treatment of antibiotic-containing wastewater.