Clinical class 1 integron-integrase gene – A promising indicator to monitor the abundance and elimination of antibiotic resistance genes in an urban wastewater treatment plant

In this study, 295 antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) from the influent, activated sludge (AS), and membrane bioreactor (MBR) permeate were primarily examined in the wastewater treatment plant (WWTP) biweekly over 13 months. The absolute concentrations of ARGs and MGEs respectively ranged from 1.27 × 10¹⁰ to 1.94 × 10¹¹ and 8.00 × 10⁹ to 1.24 × 10¹¹ copies/L in the influent, of which were reduced by 2 to 3 orders of magnitude in the permeate. No significant seasonal variation of ARGs and MGEs was found in the WWTP, except that the absolute abundance of ARGs and MGEs in the AS was peaked during spring. The antibiotics affected neither ARGs nor MGEs significantly, suggesting their concentrations may be not high enough to pose a selective pressure. In contrast, the bacterial community had direct effect on the MGEs variation, meanwhile the MGEs influenced the ARG abundance directly. Class 1 integron-integrase gene (intI1), clinical intI1, and Tn21 associated more frequently with ARGs in the AS over long-term, suggesting the potential of them involved in horizontal gene transfer. Both intI1 and clinical intI1 had significantly positive associations with the overall abundance of ARGs, as well as significantly negative relationships with the overall removal rates of ARGs in the MBR. However, the abundances between intI1 and clinical intI1 were significantly different. Meanwhile, clinical intI1 remained rather consistent proportion with the ARG abundance in the AS and permeate, was stronger correlated with human pathogens, and was associated with greater number of ARGs over time. Moreover, clinical intI1 was significantly associated with the removal efficiency of ARGs from all classes. Taken together, clinical intI1 can be adopted as an indicator for the abundance and removal efficiency of ARGs in the WWTP.