With the massive worldwide population increase, water has been predicted to become one of the scarcest resources in the 21st century. Moreover, UN’s World Water Development Report, 2017, said globally 80% of sewage (> 95% in developing countries) is directly released to the environment. Numerous questions have been raised about the ability of wastewater treatment programmes to remove pathogens from wastewater in which many waterborne diseases are associated supposedly treated water supplies. The untreated wastewater can threaten human and ecological system by enriching the pathogenic bacteria (Zhang et al., 2018). Moreover, a body of literature has confirmed the presence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in wastewater (Ahmed et al., 2020; Haramoto et al., n.d.; Zhang et al., 2020). The SARS-CoV-2 possibly spreads through wastewater treatment network (Naddeo and Liu, 2020). When raw sewage contaminates the aquatic environment, these pathogens can be transmitted through water to those who use the water for swimming, boating and fishing, causing a range of potential risks (Cornelisen et al., 2011). However, there exist many ways for untreated wastewater to enter rivers. When sewage treatment can not treat too much wastewater or when it was rains heavily, raw sewage will be discharged directly into river channels, polluting the environment and making the water quality worse. Natural disasters, such as earthquakes and floods, also make raw sewage flow into rivers, causing pathogens in rivers (Devane et al., 2014). Once sewage delivers to receiving waters, a series of physical and environmental factors changes will occur. Over time, river dilution, storage in sediments, and the intrinsic characteristics of the microorganisms and other related process may alter the destiny of the bacteria the pathogens of concern.
The direct discharge of sanitary wastewater may cause environmental diversity changes. Sewage discharge without proper treatment significantly altered the concentrations of different organic and inorganic contaminants in the receiving water bodies (Duttagupta et al., 2020), especially NH4+, NO3−, total phosphorus (TP) and chemical oxygen demand (COD). The increase of these contaminants could trigger the aquatic organism by causing eutrophication of the catchment (Huelsen et al., 2016). David et al. (2014) found that the high NH4+ concentration negatively influenced the functional performance and taxonomic richness of the microbial community of 10 different WWTPs located across Switzerland (David et al., 2014). Moreover, phosphorus content was positively correlated with the microbial biomass (Lei, 2012) and occupied an important position among environmental factors that affect the microbial community of sediments (Jian, 2015; Yu et al., 2017).
Several common infectious pathogens, such as Vibrio cholera, Staphylococcus aureus, Mycobacterium tuberculosis, and Helicobacter pylori, exist in aquatic environments. Vibrio cholerae, a well-known internationally quarantinable infectious pathogenic bacterium could cause human enteric infection (Reidl and Klose, 2002). Vibrio cholera O1 is widely distributed in aquatic environments, namely rivers, ponds, sewage, and estuaries, in many developing countries such as Haiti (Chin et al., 2010). Reidl et al. (2002) isolated Vibrio cholerae from estuarine and aquatic environments (Reidl and Klose, 2002). Staphylococcus aureus may cause local purulent infection, pneumonia, and colitis after enrichment in water (Minoru et al., 2017). Zieliński et al. (2020) found Sewage treatment plans are a sizeable source of drug-resistant staphylococci harboring virulence genes (Zieliński et al., 2020). Moreover, Mycobacterium tuberculosis normally enters the host via mucosal surfaces, usually through the lung after inhaling infectious droplets from an infected individual and occasionally via the intestine after ingesting infected material (DIETRICH and DOHERTY, 2009). Velayati et al. isolated Mycobacterium tuberculosis from water samples collected in Tehran, Iran metropolitan area (Velayati et al., 2015). Many diseases such as gastritis, peptic ulcer, and lymphoid proliferative gastric lymphoma were caused by Helicobacter pylori infection (Pereira and Medeiros, 2014). West et al. (1992) reported the capabilities of Helicobacter pylori to survive in various buffers at room temperature over a range of physical variables, which means that Helicobacter pylori may survive in a natural aquatic environment (West et al., 1992). Therefore, pathogenic bacteria in water environment will have direct or indirect effects on human beings or animals. The study research and analysis their microbial community composition and influencing factors is urgent for future treatment measure.
The lack of a thorough understanding of the survival and persistence of different microbial types in different conditions and environments is one of the major gaps in the knowledge of pathogenic microorganisms in wastewater. Previous studies focused on the abundance of pathogenic bacteria and the relationship between the contaminants and pathogenic bacteria in rivers contaminated by raw sewage discharge (McCarthy, 1996; Miller et al., 2006). However, it is unclear whether the source of pathogenic bacteria in the river comes from sewage itself or is caused by the environmental change after sewage discharge. Also, a deep insight into microbial and genetic responses to river self-purification needs further investigation. Thus, the main target of the present study is to fill in this knowledge gap by investigating the environmental parameters and the microbial community structures of different sites along the Daxin River, representing treated and raw sewage discharge points. The specific objectives of this study are to (1) provide more information about the water quality and microbial community structure before and after domestic wastewater pollution, (2) explore the composition of pathogenic bacteria community in Daxin River based on PICRUSt and KEGG database, and (3) evaluate the relationship of infectious pathogens and functional bacteria to different environmental factors.