The unique features of aerobic granule sludge contribute to simultaneous antibiotic removal and mitigation of antibiotic resistance genes enrichment

Antimicrobial resistance is now an increasingly serious threat to public health. In this study, the distribution and enrichment of antibiotic resistance genes (ARGs) in aerobic granular sludges (AGS) during sulfamethoxazole (SMX) biodegradation were examined. The removal percentage of SMX kept higher than 99 % at the loading of 750 mu g/L after the formation of AGS. Through quantitative polymerase chain reaction (qPCR) and metagenomics, we showed that higher abundance of ARGs were in flocs (143 ARG subtypes, 3.08 +/- 0.99 x 107 copies per ngDNA) and the outer AGS (111 ARG subtypes, 3.50 +/- 0.25 x 107 copies per ngDNA) than that in the inner AGS (102 ARG subtypes, 8.10 +/- 0.79 x 106 copies per ngDNA). Two mechanisms may explain the simultaneous antibiotic removal and mitigation of ARG enrichment by AGS. Firstly, dense aggregated structure of AGS and the abundant extracellular polymeric substances (EPS) play an important role in mitigating ARGs enrichment by preventing antibiotic transfer into the inner space of AGS. The deterministic mechanisms drive AGS bacterial community assemble (beta NTI > 2) in a direction toward accumulating EPS producers. Secondly, the higher abundance of antibiotic co-metabolic genes in AGS than flocs revealed that functional bacteria in the outer AGS may decrease antibiotic pressures for the inner bacteria by co-metabolic degradation of antibiotics through nitrification and denitrification. This work discovered that AGS harbor ARG-independent resistance and degradation of antibiotics, and provided a new perspective for biological wastewater treatment.