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Metagenomic Assembly Reveals Hosts of Antibiotic Resistance Genes and the Shared Resistome in Pig, Chicken and Human Feces Liping Ma, Yu Xia, Bing Li, Ying Yang, Li-Guan Li, James M. Tiedje, and Tong Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03522 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 19, 2015
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Metagenomic Assembly Reveals Hosts of Antibiotic Resistance
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Genes and the Shared Resistome in Pig, Chicken and Human Feces
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Authors: Liping Ma1, Yu Xia1, Bing Li1, Ying Yang1, Li-Guan Li1, James M Tiedje2* and Tong
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Zhang1*
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Author affiliation:
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1
Environmental Biotechnology Laboratory, The University of Hong Kong, Hong Kong
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2
Department of Plant, Soil, and Microbial Sciences, Michigan State University, United States
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Corresponding author:
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1. Tong Zhang (Associate Professor)
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Environmental Biotechnology Lab, The University of Hong Kong, Pokfulam Road, Hong Kong
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Email:
[email protected] 13
Tel: +852-28578551
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2. James M Tiedje (Professor)
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Department of Plant, Soil, and Microbial Sciences, Michigan State University, United States
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Email:
[email protected] 17
Tel: +517-355-9021
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ABSTRACT
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The risk associated with antibiotic resistance disseminating from animal and human feces is an
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urgent public issue. In the present study, we sought to establish a pipeline for annotating
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antibiotic resistance genes (ARGs) based on metagenomic assembly to investigate ARGs and
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their co-occurrence with associated genetic elements. Genetic elements found on the assembled
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genomic fragments include mobile genetic elements (MGEs) and metal resistance genes (MRGs).
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We then explored the hosts of these resistance genes and the shared resistome of pig, chicken and
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human fecal samples. High levels of tetracycline, multidrug, erythromycin and aminoglycoside
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resistance genes were discovered in these fecal samples. In particular, significantly high level of
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ARGs (7762 ×/Gb) was detected in adult chicken feces, indicating higher ARG contamination
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level than other fecal samples. Many ARGs arrangements (e.g., macA-macB and tetA-tetR) were
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discovered shared by chicken, pig and human feces. In addition, MGEs such as the aadA5-
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dfrA17-carrying class 1 integron were identified on an assembled scaffold of chicken feces, and
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are carried by human pathogens. Differential coverage binning analysis revealed significant ARG
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enrichment in adult chicken feces. A draft genome, annotated as multi-drug resistant Escherichia
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coli, was retrieved from chicken feces metagenomes and was determined to carry diverse ARGs
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(multidrug, acriflavine and macrolide). The present study demonstrates the determination of
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ARG hosts and the shared resistome from metagenomic data sets and successfully establishes the
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relationship between ARGs, hosts, and environments. This ARG annotation pipeline based on
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metagenomic assembly will help to bridge the knowledge gaps regarding ARG-associated genes
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and ARG hosts with metagenomic data sets. Moreover, this pipeline will facilitate the evaluation
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of environmental risks in the genetic context of ARGs.
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.
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INTRODUCTION
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The emergence of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs)
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has become an urgent public health issue worldwide.1 ARGs have been detected in various
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environments, including wastewater treatment plants,2 animal feces and soil,3-6 river water,7
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drinking water,8 and glaciers.9 Although antibiotic resistance is a natural phenomenon that
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predates the modern selective pressure of clinical antibiotic use,10 the aggravation of antibiotic
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resistance has been reported in diverse environments. The emergence of ARGs could
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significantly increase the spread of antibiotic resistance, especially via mobile genetic elements
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through conjugation, transduction and transformation.1 High ARG levels were frequently found
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in livestock and human feces, likely resulting from the wide application of antibiotics and the
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bacterial acquisition of antibiotic resistance mechanisms.11 Thus, in the present study, chicken,
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pig and human feces and municipal wastewater samples reported to be ARG hot spots5, 6, 12 were
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investigated.
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Recently, high-throughput sequencing-based metagenomic analysis has been used to explore
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the diversity and abundance of ARGs in various samples.6, 13, 14 In a previous study by our group,
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the non-redundant structured Clean Antibiotic Resistance Genes Database (ARDB – 25 ARG
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types and 620 ARG subtypes) was constructed and used to annotate ARG-like genes. The
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database was also used to investigate the abundance of ARGs in activated sludge sampled over a
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four-year period15 and in 50 samples from various environments.6 The same approach has also
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been used to study ARG profiles in marine sediment16 and sludge samples from different
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wastewater reactors17. These studies demonstrated that a metagenomic approach and a structured
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ARG database are powerful novel tools for the high-throughput identification and quantification
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of ARGs in different environments. However, key information such as ARG hosts, ARG co-
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occurrence, ARG co-occurrence with metal resistance genes (MRGs), and the antibiotic
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resistome shared by different environments remains unknown. Network analysis of ARGs and
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bacterial populations might provide indirect information about ARG hosts via co-occurrence
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patterns.6 Recently, several studies have begun investigating the open reading frames (ORFs) of
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ARGs on the assembled contigs of genomes from cultured strains.18, 19 These studies found that
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soil bacteria and human pathogens shared some antibiotic resistomes (e.g., tetR-tetA(G)),18
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indicating a possible exchange of ARGs between environmental bacteria and clinical pathogens.
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Furthermore, these studies demonstrated the importance of knowledge regarding the hosts of
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specific ARGs and the resistomes shared by different environments. However, the annotation of
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metagenomic raw reads cannot provide a comprehensive survey of ARG arrangements on
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bacterial genomes, ARG hosts and the resistomes shared by different environmental samples.
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In the present study, we applied an approach based on assembled metagenomic sequences to
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identify ARG-carrying contigs (ACCs) in feces (human, pig, and chicken) and municipal
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wastewater samples shown to possess high levels of diverse ARGs. We then explored the
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correlations of ARGs with hosts, integrons, MRGs and environmental niches following the
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research pipeline shown in Figure 1.
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MATERIALS AND METHODS
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Sample Collection and Sequencing. ARG profiles of different environmental samples were
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compared based on unassembled raw reads from our previous publication.6 This comparison
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illustrated the high abundance of ARGs in wastewater influents and fecal samples from chickens,
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pigs and humans. In addition, the study suggested these sources as the main sources of ARG
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contamination in the environment. To detect ARG-carrying contigs and to explore the shared
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resistome, animal (pig and chicken) feces, human gut microbiomes and wastewater treatment
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plant influents were selected from our previous study6 and reanalyzed. Basic information of the
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16 selected samples is summarized in Table S1. Chicken (20-day and 80-day) and pig (1-month
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and 8-month) fecal samples were collected from livestock farms in southern China in October
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2012. Chicken (20-day and 80-day) samples were abbreviated as CF20d and CF80d, and pig (1-
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month and 8-month) samples were abbreviated as PF1m and PF8m, respectively. The sampling
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procedure, fixation method, DNA extraction procedure, and Illumina sequencing method have
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been described previously.6 After discarding low-quality reads, sequencing depths of 2.5~3 Gb
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were obtained for each sample (Table S1). Metagenomic data for human feces (Zhuhai, southern
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China) were downloaded from the NCBI SRA (National Center for Biotechnology Information
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Sequence Read Archive) database (accession number SRX095751, SRX095764, SRX095769
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and SRX095775) and had a total size of 8.6 Gb.
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Metagenome Assembly and ORF Prediction. After quality control, reads were assembled
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using CLC Genomics Workbench (Version 6.0.2; CLC Bio, Aarhus, Denmark) with a Kmer of
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63.20 Assembled contigs longer than 500 bp were used for downstream analysis. In total, 95,042,
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108,016, 22,338, 42,665, 55,982 and 158,007 contigs (≥ 500 bp) were obtained from the CF20d,
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CF80d, PF1m, PF8m, Influent (wastewater influent from Hong Kong Shatin wastewater
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treatment plant) and HumanGut datasets, respectively (Table S2). ORFs within contigs were
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predicted using MetaGeneMark (Georgia Institute of Technology, Atlanta, Georgia, USA).21 The
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average coverage of each contig/ORF was quantified by mapping metagenomic reads to the
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contig/ORF using CLC Genomics Workbench with a minimum similarity of 95% over 95% of
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the read length.
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Identification of ARG-like ORFs. The nucleotide sequences of the ORFs were searched for
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ARGs against the non-redundant structured Clean ARDB developed by our group15 using
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BLASTX with an E-value ≤ 10−10.22 An ORF was designated an ARG-like sequence if its best
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BLASTX alignment to ARG sequences was at least 80% similar with a query coverage of at least
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70%. This identification approach was determined to be highly precise (precision of 99.1%) by
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Hu et al.22. Identified ARG-like ORFs were automatically sorted into 25 ‘ARG types’ (e.g.,
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tetracycline resistance genes) and 620 ‘ARG subtypes’ (e.g., tetA, tetB, tetC) using a package of
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customized scripts.
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Annotation of ORFs on ACCs. The presumed protein ORF sequences were compared with
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the local non-redundant NCBI NR database (downloaded on 18 July, 2013) using BLASTP with
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an E-value ≤ 10−2.23 Using a customized script, these ORFs were then annotated by a
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customized script according to the corresponding descriptions retrieved for each ORF from the
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NR database. Finally, all ORF annotations and ORF arrangements on ACCs were summarized
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and checked manually to analyze ARG distribution and co-occurrence.
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Taxonomy Annotation of ACCs. The predicted protein sequences of ORFs on ACCs were
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compared with the local NCBI NR database using BLASTP with an E-value ≤ 10−5.20 Search
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results were parsed and annotated with MEGAN (MEtaGenome ANalyzer, Version 5).24 If more
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than
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kingdom/phylum/class/order/family/genus, the contig was assigned to that taxon25 using a
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customized R program for voting. Sequences of interesting ACCs (e.g., ACCs carrying a
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combination of ARGs and associated genetic elements or ACCs shared by different
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environmental niches) were submitted to NCBI using BLASTN to obtain sequence taxonomy
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annotations at the species level.
50%
of
the
ORFs
on
a
contig
were
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the
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Sequence Composition-Independent Binning and Retrieval of the ARG-Carrying
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Genome Bin. Sequence composition-independent binning26 was conducted to compare the
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distributions of ARGs in different samples. Binning was facilitated using the metagenomes
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generated from CF20d and CF80d (chicken) and PF1m and PF8m (pig) fecal samples. After
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mapping the reads from each of the two metagenomes to the assembled contigs and plotting the
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two coverage values against each other in R, distinct contig groupings were identified.26 Contigs
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were colored according to their taxonomic affiliation and guanine-cytosine (GC) content,
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respectively. Contigs carrying ARGs were also marked with red triangle icons. The ARG-
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carrying draft genome was retrieved according to the approach proposed by Albertsen et al.26
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The completeness and potential contamination of the genome bin was evaluated using hidden
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Markov models (HMMs) of 107 essential single-copy genes conserved in 95% of bacteria.27
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Then, the ARGs on the retrieved draft genome were identified. Functional analysis of the
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genome bin was performed using KEGG (Kyoto Encyclopedia of Genes and Genomes)
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Mapper.28
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Accession Number. The assembled metagenomic genomes were deposited in MG-RAST (Metagenomic Analysis Server) under the accession numbers 4572808.3~4572813.3.
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RESULTS AND DISCUSSION
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Assembly and ACCs. After comparison against the non-redundant Structured Clean ARDB
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database using BLASTX, 52, 170, 114, 33, 108 and 156 ORFs were annotated as ARG-like
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sequences in the CF20d, CF80d, PF1m, PF8m, Influent, and HumanGut samples (Table S2),
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respectively. These ARG-like ORFs in the CF20d, CF80d, PF1m, PF8m, Influent, and
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HumanGut samples were carried by 49, 160, 107, 31, 99 and 113 ACCs (Table S3), respectively.
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In some cases, multiple ARGs were found on the same contig. For example, in the HumanGut
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sample, 49 ACCs (43% of ACCs) carrying more than 2 ARGs with an average length of 5.6 kbp
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were found. Many of these ACCs also carried genes associated with ARGs, such as MRGs,
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transposases, integrases and virulence factors (Table S3).
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ARG-like ORFs were assigned to different ARG types and ARG subtypes according to the
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structured ARG database. The abundance of a particular ARG type, indicated by coverage (in
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units of ‘times per Giga base’, ×/Gb), was calculated by summing the coverages of ARG-like
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ORFs belonging to that ARG type. Figure 2 shows the abundance of ARG-like ORFs in feces
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and influent samples for the 22 total ARG types detected. The highest ARG abundance was
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found in the CF80d sample, with a total abundance of 7762 ×/Gb. Of the ARGs detected,
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tetracycline resistance genes were the most abundant in the PF1m, PF8m, Influent and
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HumanGut samples, with abundances of 1090, 526, 115 and 525 ×/Gb, respectively. For the
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CF20d and CF80d samples, the tetracycline resistance genes were the second highest ARG type,
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with abundances of 1362 ×/Gb and 827 ×/Gb, respectively. Multidrug resistance genes were
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extremely abundant in the CF80d sample, with an abundance of 2986 ×/Gb.
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Tetracycline resistance genes, the most widespread and dominant ARGs, were detected with
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high diversity in feces and influent samples. In all, 4, 16, 8, 5, 11 and 12 subtypes of tetracycline
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resistance genes were found in the CF20d, CF80d, PF1m, PF8m, Influent and HumanGut
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samples, respectively. The CF80d sample was the most diverse, containing tetA, tetA(33),
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tetA(39), tetB, tetC, tetG, tetH, tetL, tetM, tetO, tetX, tetY, and other genes (Table S4). To further
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explore the diversity of the tetracycline resistance ARG subtype, a phylogenetic tree was
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constructed consisting of the tetA-like ORFs in these samples and the reference tetA protein
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sequences in the ARDB database (Figure S2). This tree revealed that the tetA genes in chicken
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feces, pig feces, influent and human feces were highly homologous, suggesting that homologous
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tetA genes can be captured by diverse hosts and shared between different fecal and influent
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samples.
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Host Identification of ACCs. A comprehensive taxonomic survey of ACCs was conducted to
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enhance our knowledge of 1) the hosts of these specific ARGs and 2) the types of ARGs
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commonly carried by a particular host. ACCs were first classified according to the ARG types
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each ACC carried. Then, contig taxonomies were determined by using BLASTP with the NR
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database and by annotation with MEGAN and R voting. Table S5 summarizes the taxonomy
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annotation results at the genus level for ACCs and the ARG types carried by each ACC,
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successfully establishing connections between hosts and ARGs via contigs. Only 13~28% of the
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contigs in the CF80d, PF1m, Influent and HumanGut samples could be annotated at the genus
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level. The other contigs could not be classified, indicating our limited knowledge of the ARG
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hosts in these environmental samples. This limitation is common in the taxonomy identification
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of contigs generated from metagenomic data.6 Although >30% and >50% of contigs were
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annotated at the family and phylum levels, the low annotation percentage at the genus level
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hinders the identification of ARG hosts. Despite this low annotation percentage, the Escherichia
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genus exhibited high correlation with multidrug resistance genes, which were carried by 40%,
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33%, 56% and 75% of the Escherichia-related ACCs in the CF80d, PF1m, Influent and
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HumanGut samples, respectively. These Escherichia-related multidrug-ACCs were highly
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abundant in several samples, especially CF80d (Table S6).
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Based on the widely accepted assumption that different contigs with similar coverages may
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be derived from the same species,26 the coverage values of Escherichia-related multidrug-ACCs
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were grouped into a single group using an RSD (relative standard deviation) cutoff of 23,602-, >15,477- and >15,430-fold, respectively. The total abundance of ARGs in chicken
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feces increased by 137%, while the total abundance of ARGs in pig feces decreased by 49%.
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One ARG-carrying genome bin (consisting of 390 contigs) was successfully retrieved from
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metagenome data sets using the differential coverage approach (Figure S4). The extracted
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genome bin was annotated as E. coli, with a completeness of 57% and no redundancy (Table
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S13). In addition, the abundance of this E. coli bin was increased 77-fold in CF80d compared to
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CF20d. In all, 9 out of 390 contigs carried ARGs, including 1 acriflavine ARG, 1 macrolide ARG,
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7 multidrug ARGs and 1 other ARG (Table S14). Functional analysis of this E. coli strain using
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MEGAN KEGG revealed that the strain might cause human disease and could possess antibiotic
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and metal resistance. Therefore, this E. coli strain is likely a high-risk human pathogen.
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In previous studies, the differential coverage binning of multiple metagenomes was mainly
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used to retrieve draft genomes.26 Here, for the first time, sequence composition-independent
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binning was used to explore ARG distributions in fecal samples using multiple metagenomes.
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Binning analysis showed a significant enrichment of ARGs in adult chicken feces and an
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apparent decrease of ARGs in adult pig feces. These variations are consistent with application of
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antibiotics on livestock farms: decreasing amounts of antibiotics are applied during pig growth,
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while chickens are fed antibiotics the entire time.45, 46
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Our study established a metagenome assembly-based method for identifying ARG-carrying
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contig/genomes in environmental samples. This method successfully answered several questions
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that could not be resolved using traditional PCR/qPCR approaches or short read-based
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metagenomic analysis. We used this approach not only to identify ARGs on contigs but also to
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quantify the abundance of ARGs in samples based on coverage, to identify the hosts of ACCs, to
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explore the shared antibiotic resistome among bacteria, and to determine the correlations of
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ARGs with associated genetic elements. In the future, this approach could facilitate antibiotic
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resistance analysis using metagenomic data sets to explore the ARG distributions in various
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environmental samples and could expand our horizons on the presence, co-occurrence and hosts
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of ARGs. Finally, this approach could greatly benefit environmental risk assessments of ARGs.
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ASSOCIATED CONTENT
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Supporting Information
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Additional information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
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*Telephone: +852-2857-8551. Fax: 0-852-2559-5337. E-mail:
[email protected]. (Tong Zhang)
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*Telephone: +517-355-9021. Fax: 517-353-2917. E-mail:
[email protected]. (James M Tiedje)
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Notes
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The authors declare no competing financial interest
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ACKNOWLEDGMENTS
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This study was financially supported by the Hong Kong GRF (HKU17209914E). Liping Ma
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thanks The University of Hong Kong for the postgraduate studentship. Dr. Yu Xia and Bing Li
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thank The University of Hong Kong for their postdoctoral fellowships. Dr. Ying Yang thanks The
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University of Hong Kong for the research assistant fellowship. The work at MSU was supported
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by the Center for Health Impacts of Agriculture (CHIA).
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Figure Legends Figure 1. The analysis pipeline of this study. Figure 2. The abundance (coverage of ARG-like ORFs, ×/Gb) of ARG-like ORFs in samples. MLS: Macrolide-lincosamide-streptogramin.
Abundance (coverage,×/Gb) =
#N mapped reads $
× 100/L S
!"
Where Nmapped reads is the number of the reads mapped to ARG-like ORFs, LARG-like ORF is the sequence length of the corresponding target ARG-like ORF sequence, n is the number of ARG-like ORFs belonging to the same ARG type, 100 is the sequence length of the Illumina reads, and S is the size of the dataset (Gb). Figure 3. The arrangements of shared multiple-ARG-carrying contigs and their putative hosts (best hits on NCBI) in different samples. Figure 4. A complete class 1 integron (carrying aadA5 and dfrA17) detected on contig CF20d_5145 (4765 bp) and annotated as a fragment of Escherichia coli O25b:H4ST131 str. EC958 (HG941719.1) and Salmonella enteric subsp. enterica serovar Newport str. USMARC-S3124.1 (CP006631.1) by NCBI BLASTN (best hit with a query coverage of 86%, an e-value of 0.0, and an identity of 100%). Figure 5. The distribution of ARG-carrying contigs in chicken metagenome datasets via differential coverage-based binning. All nodes represent scaffolds, scaled by the square root of their length, and colored by community taxonomy.
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Figure 1. The analysis pipeline of this study.
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Figure 2. The abundance (coverage of ARG-like ORFs, ×/Gb) of ARG-like ORFs in samples. MLS: Macrolide-lincosamide-streptogramin. Abundance (coverage,×/Gb) = Where, Nmapped
reads
#N
mapped reads
$
× 100/L S
!"
is the number of the reads mapped to ARG-like ORFs, LARG-like
ORF
is the
sequence length of the corresponding target ARG-like ORF sequence, n is the number of the ARG-like ORFs belonging to the same ARG type, 100 is the sequence length of the Illumina reads, and S is the size of the dataset (Gb).
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Contig ID (length)
Host
Arrangement protease ATP-binding subunit ClpA cold-shock protein CspD
(100% coverage, 99% identity)
macA macB
Escherichia coli
macA
(100% coverage, 99% identity)
macB
Escherichia coli HumanGut_13041 (96% coverage, 100% identity) (3,698 bp) Influent_46410 (1,100 bp)
Escherichia coli
Influent_2944 (996 bp)
macA
macB
transposase
(98% coverage, 99% identity)
HumanGut_13130 (3,894 bp)
PF1m_8410 (3,476 bp)
macB
Escherichia coli
Virulence factor virK
macB
(100% coverage, 99% identity)
CF20d_4856 (5,890 bp)
CF80d_26857 (3,327 bp)
Others
ATP-dependent Clp protease adaptor protein ClpS
(100% coverage, 99% identity)
Influent_28755 (1,036 bp)
MRG
macB
Escherichia coli
PF1m_14709 (1,503 bp)
MGE macA
macB
Escherichia coli
CF80d_574 (4,277 bp)
ARG
tetR
transposase
tetA
multidrug transporter
transposon Tn1721 resolvase
tetR
Proteus mirabilis
tetA
amidase Tn1721 transposase
multidrug transporter isochorismatase transposase
(99% coverage, 99% identity) arsenic transporter Corynebacterium glutamicum arsR
hypothetical protein
tetR
tetA
(42% coverage, 99% identity)
Chlamydia trachomatis
transposase tetR
phage integrase family protein
tetA
(96% coverage, 99% identity) conjugative transfer ATPase
Acinetobacter baumannii
tetR
tetA
(100% coverage, 100% identity) integral membrane protein tetracycline repressor protein homolog Trh
CF20d_7850 (1,942 bp)
Corynebacterium glutamicum
CF80d_9370 (2,733 bp)
Corynebacterium glutamicum
PF1m_11930 (3,929 bp)
Corynebacterium glutamicum
tetR
tetA
mycinamicin resistance protein homolog MyrA
(100% coverage, 99% identity) mycinamicin resistance protein homolog MyrA
tetA
hypothetical protein
(83% coverage, 100% identity)
tetA
mycinamicin resistance protein homolog MyrA
(99% coverage, 99% identity)
Figure 3. The arrangements of shared multiple-ARG-carrying contigs and their putative hosts (best hits on NCBI) in different samples.
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CF20d_5145 (4765 bp) GCN5-like N-acetyltransferase 0
1000
qacEdelta1 sul1
aadA5 2000
dfrA17
3000
intI1 4000
4765 (bp)
Class 1 integron ARG MGE Others
Length of 86% of the contig Identity of 100% aligned to two strains
Escherichia coli O25b:H4-ST131 str. EC958 (HG941719.1) Salmonella enteric subsp. enterica serovar Newport str. USMARC-S3124.1 (CP006631.1)
Figure 4. A complete class 1 integron (carrying aadA5 and dfrA17) detected on contig CF20d_5145 (4765 bp) and annotated as a fragment of Escherichia coli O25b:H4ST131 str. EC958 (HG941719.1) and Salmonella enteric subsp. enterica serovar Newport str. USMARC-S3124.1 (CP006631.1) by NCBI BLASTN (best hit with a query coverage of 86%, an e-value of 0.0, and an identity of 100%).
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Figure 5. The distribution of ARG-carrying contigs in chicken metagenome datasets via differential coverage-based binning. All nodes represent scaffolds, scaled by the square root of their length, and colored by community taxonomy.
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