Unban water systems (UWSs) are designed to evacuate wastes from the human activity area (such as hospitals and residential places) to low human exposure areas and gradually reinstate them into natural watercourses. Meanwhile, abundant and diverse ARGs are also leaked and carried by the flow due to the globalization and widespread of antibiotic resistance genes (ARGs) in the natural environment and clinical settings, which has become one of the biggest health concerns in the current century 1. UWSs as the unique conduit between humans and environments are indisputable hotspots of horizontal gene transfer (HGT) for the spread and evolution of ARGs 2, 3, 4, 5. Mobile genetic elements (MGEs), especially plasmids and integrative conjugative elements (ICEs), are the primary vectors disseminating ARGs among close and/or remote phylogenetically related microbes, as well as causing the accompanying multidrug resistance (MDR) issues in the problematic nosocomial pathogens of the ESKAPEE group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp., and Escherichia coli) 6.
Extensive studies have been paid to uncover the antibiotic residues, ARGs and antibiotic resistome (i.e., the entire set of ARG populations 7) in the wastewater treatment plants (WWTPs) 5, 8, 9. And some researchers forethoughtfully explored the linkage among the resistome, plasmidome (all plasmids within a given environment or microbial community 10) and the microbiome in activated sludges from the WWTPs 11, 12. Much work has been done to determine the specific resistome features in a particular treatment chamber in the WWTPs, especially the biological treatment processes (BTPs), which provide helpful advice for the wastewater risk management and further operational modification 13, 14, 15. A nine-year longitude temporal metagenomic study deciphers the consistent relationship between the ARGs and MGEs in the activated sludges 16, although long-term characterizations of the sludges reveal variable changes in the microbiome composition 17, 18. Meanwhile, Yin et al. compared the resistome and mobilome (i.e., all MGEs in a studied microbiome 19) of the influent, activated sludge and effluent samples from different WWTPs with the purpose of assessing the spatial variations within the WWTPs 20. In general, the aforementioned studies comprehensively investigate the composition and distribution of the antibiotic resistome and mobilome in the WWTPs or receiving rivers, whereas lacking a full picture of taking the whole UWS into consideration, for instance, the upstream sewer catchments.
In this study, we used our previously documented UWS plasmidome dataset, which applied a direct plasmid DNA metagenome sequencing approach to assess different treatment stages in three distinct UWSs of three European countries 21. We benchmarked our rigorous experimental methodology for harvesting almost pure plasmid DNA from the sewer/wastewater samples 21. Accordingly, we believed the assembled circular and linear contigs were all plasmids (at least predominantly, there may be other MGEs included), while the linear contigs were not circularized due to sequencing depth or intrinsic unique genetic contexts. Then we removed the chromosome noise in the dataset, and the remaining pool of the contigs constituted the target plasmidome in this study. Through in silico analysis, we showed the dynamic plasmidome derived mobile resistome compositions, abundances, and distributions from sewer sources [hospital sewer (HS) and residential sewer (RS) lines], to the end of sewer lines (also known as the mixed sewer, i.e., MS, was often regarded as the WWTP influent), and finally in the BTP of the WWTP (Fig. 1A). Meanwhile, we detected the common ARGs shared by the three UWSs and inspected the epidemic origins of the ARGs emerging in BTPs by source tracking. Further, we recruited the microbiome and high-throughput qPCR datasets of the same sampling campaign 22. Accordingly, the unrevealing correlations among the plasmid-mediated antibiotic resistome, microbiome and the ‘total’ resistome were demonstrated. Overall, this study contributes to the monitoring of antibiotic resistome in the perspective of high mobility for transmission and evolution, and a manifest understanding of the mobile gene reservoir that potentially originates ARGs dissemination in the UWSs.