Behavior Of Antibiotic Resistance Genes In The Wastewater Treatment System And Disinfection Process

The abuse of antibiotics promotes the development of antibiotic resistant bacteria（ARB）and antibiotic resistance genes（ARGs）,which shade risks to public health.Wastewater treatment systems（WWPTs）have been considered as an important reservoir and environmental suppliers of ARGs.ARGs and ARB were detected in many units of wastewater treatment processes.Moreover,WWPTs have also been proposed to be hotspots for horizontal gene transfer of ARGs,enabling the spread of ARGs between different aquatic bacteria.Therefore,further study on the behavioral characteristics of ARGs in WWPTs is of great significance for the effective control of ARGs.In this study,the occurrence and distribution of five tetracyclines ARGs（tetA,tetC,tetG,tetQ,tetS）,class I integron（intI1）and 16S rRNA in WWPTs were studied.The influence of chlorination conditions on the horizontal transfer of ARGs and the underlying mechanism were studied.The results showed that the five-selected tetracycline ARGs and class I integrons in each process of the treatment system of the WWTP had high detection rates and high detectable concentrations.The total reduction of ARGs by the wastewater treatment system was 5.30 log.The removal efficiencies of ARGs,by advanced treatment process such as flotation and reverse osmosis,were higher than primary and secondary sedimentation tanks.Ultrafiltration and chlorine disinfection treatment only part of the reduction of ARGs.But the copies of ARGs were increased by 0.13 log after oxidation ditch.There were significant positive correlations between ARGs with the total amount of microorganisms or type I integrons.In chlorination,there was no significant deactivation of ARB with the range of0-0.2 mg/L（as Cl₂）,and there was no significant inhibition of the frequency of ARGs transfer.However,with a chlorine dosage of 0.4-0.8 mg/L,the number of conjugative transformants and the frequency of ARG transfer mediated by RP4 plasmid were decreased.The conjugative transfer frequency of ARGs between Escherichia coli was promoted by the presence of natural Bromide（Br^-）or（I^-）Iodide in chlorination.Br^-or I^-which combined with chlorine disinfectant,promot ARGs horizontal transfer.The presence of Br^-or I^-in chlorination improved mRNA expression of oxidative stress-regulatory gene（rpoS）,outer membrane protein gene（ompC）,global regulatory genes（korA,korB,trbA）and conjugation-relevant genes（trbBp,trfAp）.