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/Water 6()114979 showed that leaf biofilm influences the utilization of leaf leachate Pantoea rwandensis ●● Pantoea rodasi ● ● Pantoea vagans Pantoea agglome ans ● echangetARCSbenweeeiniealpthogesndentonmenti Leclercia adecarboxylata Pse nonas oryzae niv ● Pseudomonas chl Pse hizosp nas synnga nce Pse mendo ont plastic biofilm indeed out which these tha Pseud as(17.78% f th domonas grami fomonas grimon Pseudomonas pel 00000000000000o0 minant host was ona Most of th Pseudomonas cichon Pseudomonas coronafaciens p05 and am Pseudomonas sagittana ● e host Pseu nas in the Pseudomonas linyingensis ● Pseudomonas abietaniphila c biofiln Pseudomonas fluorescens ●● hree see R MPL host track h 6 major hosts (Pr at the 1.an w of th n.but g.8 rtunisti pathogen n clinical e ator in ivity and blag for s (Ly 0a2010 et al. with high residenc e time under natural conditions.Microplastics opportunistie, ohabitat for biofilm path atin the n posure to the water area.A num 2013na hethe the archi comple whether the The plant pathogens carrie by micropl may lea d to pe the path ens in the biofilm com and have an impact c treating the infection showed that leaf biofilm influences the utilization of leaf leachate (McArthur et al., 1985). The highest diversity of ARG (number of ARG subtype) in the leaf biofilm reminded us that leaves may be a natural source of ARGs in the river environment. In addition to microplastics, we should also focus on the potential to transfer ARGs inside the freshwater system. A previous study based on a metagenomics analysis showed the exchange of ARGs between clinical pathogens and environmental bacteria (Forsberg et al., 2012). After verifying that the microplastic biofilm loads ARGs, it is essential to trace back the origin of the ARGs and to identify which ARGs were carried by which bacteria. 3.5. Microplastic biofilm selectively enriched opportunisitic human pathogens which carried ARGs Biofilms equip the microbial community with novel functions, including increased resistance to antibiotics (Stewart and Costerton, 2001). After verifying that the bacteria in the micro￾plastic biofilm indeed possessed genes with resistance to antibi￾otics, questions remain about which bacteria possess these ARGs and whether they pose a threat to human health. To answer that question, taxonomic annotation of ARG bacterial hosts was con￾ducted. At the genus level, we found that with the exception of the unclassified Pluralibacter (18.97%), Pseudomonas (17.78%), Leclercia (11.71%), and Pantoea (11.06%) are relatively abundant in all of the samples (Fig. 7A) (Table S8). The proportions of the four genera in biofilm on three types of substrates are shown in Fig. 7B. In both microplastic and rock biofilms, the dominant host was Pseudo￾monas with a proportion of 34.7% and 26.7%, respectively, while the largest proportion (20.9%) in leaf biofilm was Pantoea. Most of the hosts possessed multidrug resistance genes and aminoglycoside resistance genes (Fig. 7C). The multidrug resistance genes were carried by 50.9%, 50.2%, 46.8% of the host Pseudomonas in the microplastic biofilm, rock biofilm and leaf biofilm, respectively (Table S9). The presence of Pseudomonas in the microplastic biofilm was further verified by quantitative real-time PCR of samples from three independent bioreactors (see supplementary material for detailed data Table S10, Table S11). Next, host tracking analysis of the four major hosts (Plural￾ibacter, Pseudomonas, Leclercia, and Pantoea) was conducted at the species level. A number of the scaffolds carrying ARGs were an￾notated to plant pathogens and human pathogens. It should be noted that several pathogenic bacteria were only detected in microplastic biofilm, but not in the rock or leaf biofilms (Fig. 8). These pathogenic bacteria included: Pseudomonas monteilii, an opportunistic pathogen more frequently detected in clinical en￾vironments and the major microbial species behind the disease of hypersensitivity pneumonitis and exacerbating bronchiectasis (Aditi et al., 2017; Bogaerts et al., 2011; Ocampo-Sosa et al., 2015); Pseudomonas mendocina, an environmental bacterium causing opportunistic nosocomial infections, such as infective endo￾carditis and spondylodiscitis (Chi et al., 2005; Mert et al., 2007); and Pseudomonas syringae, an agriculturally important plant pathogen (Hirano and Upper, 2000). The microplastic biofilm selectively enriched these pathogens, but no enrichment was observed in either rock or leaf biofilms. As one of the most common plant pathogens, P. syringae has more than 60 pathovars and infects almost all economically important crop species (Xin et al., 2018). In a previous study, the outbreak of harmful Alex￾andrium taylori algae bloom was shown to result from the attachment of A. taylori cells to plastic debris (Maso et al., 2003). The plant pathogens carried by microplastics may lead to pecu￾niary loss. The existence of pathogens with ARGs will increase the risk of antibiotic ineffectiveness because of the difficulty in treating the infection. Organic aggregates in water are islands for microbial assem￾blages and sometimes for pathogens (Lyons et al., 2010). Similarly, microplastics could be considered as a much more durable island with high residence time under natural conditions. Microplastics in the river water environment may become the preferred surface for specific pathogens with other alternative natural particles existing in the same environment at the same time. Microplastics not only serve as a novel microhabitat for biofilm colonization, but they also increase the likelihood of pathogens propagating. The pathogens may have contact with humans along the river via direct exposure to the water area. A number of questions remain to be addressed by future research, including whether the architecture of the biofilm will remain complete after longer migration distances and greater geographic spread, whether the microbial community structure will be continuously shaped because of environmental factors deviation in varied water environments, and whether the pathogens in the biofilm remain hazardous and have an impact on local microbial communities. Fig. 8. Taxonomic assignment of ARG host profiles in the microplastic, rock, and leaf biofilms at the species level. The dots coloured if the host species was detected in the corresponding biofilm. The host species was coloured yellow when it is detected in the rock biofilm, red when it is detected in microplastic biofilm, green when it is detected in the leaf biofilm), grey if it was not detected. R stands for rock biofilm; MP stands for microplastic biofilm; L stands for leaf biofilm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 10 X. Wu et al. / Water Research 165 (2019) 114979
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