Anaerobic digestion reduces extracellular antibiotic resistance genes in waste activated sludge: The effects of temperature and degradation mechanisms

The fate of extracellular antibiotic resistance genes (eARGs) in waste activated sludge during anaerobic digestion (AD) remained unclear. The current study investigated the changes in seven eARGs (sulI, sulII, tet(A), tet(O), tet (X), blaTEM, and blaSHV) and intI1 during sludge AD at 35 degrees C and 55 degrees C. First, the extracellular DNA (eDNA) extraction method from sludge was optimized by adding sodium dodecyl sulfonate, and the eDNA recovery nearly doubled. Second, analysis via qPCR showed that eARGs ranged from 1.5% to 85.1% of the total ARGs, stressing the importance of eARGs in sludge. Besides, the abundances of all detected eARGs decreased following AD, where removal rates ranged from 22.8% to 93.9% at 35 degrees C and 52.7% to 96.6% at 55 degrees C. Further analysis showed that the removal rates of eARGs were negatively correlated with their initial abundances (P < 0.05). Last, the degradation characteristics of eARGs under AD conditions were determined. The first-order degradation rate constants for different eARGs did not vary significantly, indicating that gene sequences did not cause a removal distinction, and fitted Michaelis-Menten equation confirmed the higher eARGs degradation ability at the higher temperature. Overall, this study firstly uncovered the decrease of eARGs in sludge during AD treatment, and advanced the understanding of the positive effect of AD on eARGs dissemination control.