Efficient removal mechanism for antibiotic resistance genes from aquatic environments by graphene oxide nanosheet
Introduction
As one of the most important medical discoveries in the 20th century, antibiotics are indispensable for both treating and preventing infectious diseases in clinical medicine and are used broadly in animal husbandry to obtain high-yield and healthy livestock products [1], [2], [3], [4]. Excessive use of antibiotics has led to the emergency and spread of antibiotic resistance genes (ARGs) in the environment [1], [5]. ARGs have been widely detected in various environmental contexts such as surface [6] and ground water [7], sewage or municipal wastewater [8], [9], and even drinking water [10]. Once present in the environment, ARGs may persist over many generations, even in the absence of selective pressure [11], which could enable dissemination of antibiotic resistance traits throughout non-resistant bacteria via transfer [12] (i.e. by cell division) as well as horizontal gene transfer (HGT including conjugation, transduction, and natural transformation) [4]. Intact DNA molecules may be either integrated into genome or stay extrachromosomal elements in the form of plasmid during the transformation process in aquatic environment [13]. Extracellular ARGs (eARGs) with genetic mobile platforms such as plasmid or chromosomal DNA in water have gained increasing attention for their durability and contribution to the spread of antibiotic resistance [11], [12], [13]. Recent evidence suggests that natural bacterial transformations by short DNA fragments are more frequent in the environment [14], suggesting free DNA in water would pose a serious threat to health care for human and ecosystem. Compared with linear chromosomal, plasmid DNA with a stiffer conformation is more readily adsorbed to soil particles in the presence of divalent cations [15]. However, very little is known about the adsorption behavior and removal performance of nanomaterials as effective adsorbents toward eARGs including plasmid or chromosomal DNA in water.
Compared with traditional contaminants, ARGs are very difficult to be effectively eliminated [16], [17], [18]. It is known that wastewater treatment plants (WWTPs) play a crucial role in removal of many common contaminants, such as heavy metals, organic matter, antibiotics and pathogenic microorganisms [9], [13], [14]. For ARGs, it is recently found that although the absolute abundances of ARGs (89.0–99.8%) were reduced, however, considerable ARGs levels [(1.0 ± 0.2) × 103 to (9.5 ± 1.8) × 105 copies/mL] were still detected in WWTP effluent [19]. Du and his coworkers confirmed that biological treatment units in WWTPs could promote bacterial growth and genetic exchange, which may lead to further ARGs proliferation [20]. Some efforts have been paid to examine the performance of traditional treatment technologies (chlorination and ultraviolet irradiation (UV)) on the inactivation and removal of ARGs, which seems to be unsatisfactory and inefficient. For example, Yuan et al. found that the removal level of erythromycin and tetracycline resistance genes by chlorination was only 0.42 ± 0.12 log and 0.10 ± 0.02 log, respectively [18]. Additionally, chlorination is not environmental friendly and sustainable due to the production of carcinogenic disinfection by-products (THMs, HAAs, HANs) [21]. Zhang demonstrated that the maximum log reduction of tetX and 16S rRNA genes were 0.58 and 0.60 in the UV irradiation experiment (249.5 mJ/cm2 UV dosage), indicating the low removal efficiency of UV irradiation for removing ARGs [22]. Consequently, it is urgent and interesting to explore novel and effective technologies to remove ARGs from aquatic environments.
Graphene oxide (GO) nanosheet with abundant oxygen-containing groups including hydroxyl, epoxide and carboxyl groups is a highly oxidative form of graphene achieved by chemical exfoliation of graphite [23]. GO maintains superior properties of the 2D structure, aromatic plane and large surface area comparing with graphene [24]. It is an effective adsorbent for various environmental contaminants, such as organic pollutants [25], [26], [27], heavy metals (Pb2+, Hg2+, Cd2+ and Co2+) [28], and pathogens such as E. coli and S. aureus [23]. Although GO nanosheet as a desirable adsorbent has been extensively discussed previously, to our best knowledge, the feasibility of GO for ARGs contaminated water treatment has not been investigated.
The overarching objective of this study was to improve the understanding of GO as a potential adsorbent and the removing mechanisms of GO for ARGs in aqueous solution. Four antibiotic resistance genes (tetA, sul2, ermB and ampC) were selected as target genes due to their high concentrations in aquatic environment compartments [29], [30]. Specifically, survey such as that conducted by Guillaume have shown that tetA (against tetracycline) is commonly detected in the microbial community originating from activated sludge of hospital and urban wastewater treatment facilities [31]. The gene sul2 and ampC, against sulfonamide and β-lactams, are plentiful in many metagenomes from human and natural microbiomes [30]. The gene ermB (against macrolides), which is one of the most common erythromycin resistance genes in the environment, has been recently found on a conjugative plasmid [32]. Meanwhile, two basic types of ARGs: cyclic-ARGs (c-ARGs) and double-stranded-ARGs (ds-DNA) were applied in a comparative analysis of removal efficiency. The primary mechanism of GO removing ARGs based on the π-stacking interactions and chemical binding of functional groups between ARGs and GO by adsorption kinetics, isotherms models, electrochemical, and spectroscopy (Raman and DRIFTs) analysis. The effect of conditions of the procedure (i.e., solution pH, initial concentration of adsorbates, incubation time, temperature, and cation species) on the removal performance was also investigated. Furthermore, natural water sample was explored to validate the applicability and efficiency of GO nanosheet, suggesting excellent removal efficiency toward ARGs.
Section snippets
Synthesis and characterization of GO nanosheet
GO nanosheet were synthesized based on the modified Hummers method described previously [23]. The detailed process and reagents used are presents in the SI-Section (S) 4. The GO surface morphology is measured by transmission electron microscopy (TEM). The surface elements, functionalities and oxygen content of GO were detected by X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The specific processes of the structural
Initial concentrations of ARGs and GO nanosheet
It is generally accepted that original ARGs concentration may affect the removal efficiency [22], [36]. In this work, the adsorption of ARGs was negatively correlated with their initial concentrations (p < 0.05) (Fig. S5), and the detailed correlation coefficients were listed in SI (Table S3). It is clear that the slopes (α) of ermB and ampC (both of c- and ds-ARGs) are significantly lower than those of tetA and sul2, indicating the less effect of ARG concentration on the adsorption of ermB and
Conclusions
The removal efficiency and mechanisms of four ARGs by GO nanosheet were systematically investigated in this study. The result of kinetics showed that adsorption equilibrium can be achieved within 15 min, indicating the high efficiency of GO in the removal of ARGs. The different configuration of ARGs (c-ARGs and ds-ARGs) exhibited distinct adsorption patterns on GO due to their different molecular structures. The adsorption of c-ARGs was a base composition-dependent process, while that of ds-ARGs
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China as general projects (21377061, 21677080 and 81270041), Independent Innovation fund of Tianjin University (2015XRG0020), Key Laboratory of Colloid and Interface Chemistry (Shandong University, Ministry of Education) (201401), and Natural Science Foundation of Tianjin (Grant No. 15JCYBJC48400).
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