Multiple antibiotic resistance genes distribution in ten large-scale membrane bioreactors for municipal wastewater treatment
Introduction
Antibiotics have been widely used in medicine and agriculture settings since last century. However, only a small portion can be metabolized by humans and animals; the rest are released into the environment (Uyaguari et al., 2011, Wei et al., 2014). The accumulations of antibiotics have boomed the antibiotic resistance genes of bacteria (Zhu et al., 2013). Recently, more and more related genes have been detected in the settings, accompanied with the appearance of a few super recalcitrant antibiotic-resistant bacteria (Huang et al., 2012, Marti et al., 2014). The emergence of antibiotic resistance genes together with the mutation of microbes has become not only a public health problem, but also an immeasurable risk to the ecological stability. Wastewater treatment plants (WWTPs) are considered as a major sink which collects all the unmetabolized antibiotics from human activities (Aydin et al., 2016, Qiu et al., 2013). In addition to the incomplete degradation of antibiotics in WWTPs, the surplus sludge and effluent with the enriched antibiotic resistance genes are also a potential threat to environment (Chen et al., 2015, Gao et al., 2012, LaPara et al., 2011, Naquin et al., 2015, Rodriguez-Mozaz et al., 2015). It is critical to obtain a better understanding of the ecology of the antibiotic resistance genes in WWTPs and it will be consequently helpful to predict and counteract the negative impact that these genes might impose on ecology. The current studies on antibiotic resistance genes in WWTPs mostly confined to the analyses of several common antibiotics and corresponding antibiotic resistance genes such as tetracycline, erythromycin and sulfonamide resistance genes (Aydin et al., 2015, Munir et al., 2011). However, it is crucial to cover multiple antibiotics resistance genes because the microbial multiple antimicrobial resistance mechanisms involving the multidrug resistant efflux systems and β-lactamase have attracted global health concerns (Marquez, 2005).
The objective of this study was to illustrate the distribution and abundance of the major antibiotic resistance genes in large-scale WWTPs. A high-throughput array GeoChip was employed to analyze the antibiotic resistance genes of ten large-scale WWTPs in China. GeoChip covers four multidrug efflux system genes and three β-lactamase genes (Classes A–C), providing more insightful information than conventional microbiology approaches. All the selected ten plants are featured as a nutrient-removal process combined with a membrane bioreactor (MBR), which is commonly operated at longer sludge retention time (SRT) and higher sludge concentrations. These unique conditions may affect the development and proliferation of the antibiotic resistance genes and the corresponding microorganisms. This study focused on the overall distributions of antibiotic genes by a common wastewater treatment process, but from different geographic regions. The obtained information can provide helpful knowledge to estimate the environmental risk of antibiotic resistance genes.
Section snippets
MBRs and physicochemical analysis
The ten selected large-scale WWTPs are located in four different cities from China: Beijing (B1, B2, B3 and B4), Wuxi (W1, W2, W3 and W4), Shiyan (S1) and Kunming (K1) (Fig. S1). These plants share a similar nutrient-removal anaerobic–anoxic–oxic process combined with a membrane bioreactor (A1/A2/O-MBR). The technical processes, operational parameters and performance of the ten plants were summarized in Supporting information (Tables S1, S2). Three replicate sludge samples were collected from
Diversity of antibiotic resistance genes
The ten MBRs displayed variations of the detectable antibiotic resistance genes in terms of gene number and diversity indices. A total of 585–1305 antibiotic related genes were detected in these plants (Table 1), with the highest gene number was found in W2, followed by W1, B1 and B2; while W4 and B4 showed the fewest antibiotic gene number. According to the GeoChip 4.0 database, seven antibiotic resistance gene groups were all detected in the ten MBRs, including ATP-binding cassette (ABC),
Conclusion
The dominant multiple antibiotic resistance gene groups were different among MBR plants. Multiple antibiotic resistance genes were mostly from Proteobacteria and Actinobacteria. The TN, TP and COD of influent, temperature and conductivity of mixed liquor were the major factors that affected the distribution of multiple antibiotic resistance genes in MBRs.
Acknowledgements
This work was supported by the Key Program of the National Natural Science Foundation of China (No. 51238004), and the Program for Outstanding PhD thesis of Beijing (No. 20131000305).
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