The Fate And Transfer Mechanisms Of Antibiotic Resistance Genes During Bioelectrochemical Treatment Of Chloramphenicol Wastewater

The consumption of antibiotics has contributed to continuous emissions of antibiotics into tihe environment. The antibiotics in the environment resulted in the spread of antibiotic resistance bacteria (ARB) and antibiotic resistance genes (ARGs), which could pose a threat to human health. In typical wastewater treatment plants, antibiotics are only partially eliminated and the ARGs are produced during the biodegradation of antibiotics, which are further released into the environment. Chloramphenicol (CAP) is a broad-spectrum antibiotic that has been widely used, but it can be carcinogenic and genotoxic for humans. Due to the biotoxicity of nitro and chlorine substituents, CAP is resistant to conventional biological wastewater treatment processes. As a promising alternative, the bioelectrochemical system (BES) with biocatalyzed cathodic reduction can significantly enhance the removal of chlorine substituents and convert the nitro groups to amino substituents, which will reduce its biotoxicity and increase its biodegradability. The operating parameters of BES have a potential influence on the occurrence and abundances of ARB and ARGs. Therefore, it is of major significance to investigate the fate of ARB and ARGs in the BES. There are two main ways for ARGs transfer: vertical transfer and horizontal transfer. However, the transfer mechanism of ARGs during the treatment of CAP wastewater in BES is still unclear.In this work, a BES reactor was developed for CAP removal. The CAP removal capability of BES was examined. The fate of ARGs in BES was determined. The contributions of the shift of hosts and horizontal transfer for ARGs variations were investigated based on the correlation between microbial community (determined by Illumina Hiseq sequencing technology), the abuandance of class 1 integron integrase encoding gene (intll) and ARGs, and thus determined the mechanisms of ARGs transfer. The main conclusions are as follow:(1) In this study, we investigated the removal of CAP in BES, the degradation pathway of CAP, as well as the fate of chloramphenicol resistance bacteria (CRB) and chloramphenicol resistance bacteria (CRGs) during CAP degradation in the BES. Both initial CAP concentrations and cathode potentials can influence the removal efficiency of CAP in the BES. The activity of bacteria without resistance to CAP under more negative cathode potential (-1.25 V) would be inhibited, which may be harmful to the long-term running for BES. And the expression of CRB under less negative cathode potential (-0.5 V) were induced. Furthermore, CAP could be dechlorinated under an intermediate cathode potential (-1 V) while the abundances of CRB as well as relative expression of CRGs were abated under this condition, whichwould be favorable to mitigate the pollution of antibiotic resistance genes during the treatment of antibiotic wastewater in BES.(2) On the basis of the aforementioned result, the mechanisms of ARGs changes and transfer were determined based on the. correlation between microbial community, the abundance of intI1 gene and ARGs. The shift of ARGs potential hosts, rather than horizontal gene transfer was the major factor for the alteration of ARGs (except sul1) under different salinities and cathode potentials in this study. However, under different temperatures, both shift of ARG potential hosts and horizontal gene transfer contributed to the variation of ARGs. Furthermore, the degradation of CAP varied under different conditions and the microbial community shift was an important factor for the alteration of CAP removal.(3) Coupling of electrochemical system into the anaerobic reactor was developed. The CAP removal in the system and the fate of ARGs, the mechanisms of ARGs transfer during CAP removal in the system were investigated. Furthermore, the effects of antibiotic resistance on fuctional bacteria and functional genes were determined based on the correlation between microbial community, and ARGs. The results showed that the shift of microbial community was the main contributor for ARGs (except sull gene) changes, rather than horizontal gene transfer. Therefore, the change of the microbial community resulted in the ARGs variations, thus affected the functional bacterial abundance, and influenced the performance of the anaerobic reactor coupled by the electrochemical system.