Chapter 10 Illumina Whole Genome (Shotgun) Metagenomics with Centrifuge
10.1 What is Centrifuge?
Centrifuge is very rapid and memory-efficient system for the classification of DNA sequences from microbial samples, with better sensitivity than and comparable accuracy to other leading systems. The system uses a novel indexing scheme based on the Burrows-Wheeler transform (BWT) and the Ferragina-Manzini (FM) index, optimized specifically for the metagenomic classification problem[1].
10.2 Tutorial
For this tutorial we will use data from this publication “Metagenomic Characterization of the Human Intestinal Microbiota in Fecal Samples from STEC-Infected Patients” [2]: https://www.frontiersin.org/articles/10.3389/fcimb.2018.00025/ All data used in this tutorial was retrieved from SRA with fastq-dump. SRAccession: PRJEB23207
In this study, whole genome metagenomic sequencing was used to investigate possible changes in the composition of the intestinal microbiota in samples from patients with Shiga toxin-producing E. Coli (STEC) infection (N = 2) compared to Crohn’s Patients (N =4), healthy (N = 2) and healed controls (N = 3). Faeces samples, collected during an outbreak, from STEC infected patients showed a lower intestinal abundance of beneficial microorganisms in comparison to controls where those microorganisms predominated. These differences were observed with bioinformatic approaches and seemed to be related with the STEC infection. So, using the metagenomics data from this study can you identify the STEC positive samples? Can you identify the specific strain of STEC? Many of the steps have been done for you. However, I have provided instructions and code so that you can build the full database indexes that we will use for classifying reads with Centrifuge.
10.5 Retrieve RefSeq and EuPathDB Data
Centrifuge provides a few scripts that will allow you to download the NCBI taxonomy tree and RefSeq reference genomes.
• To map sequence identifiers to taxonomy IDs, and taxonomy IDs to names and their parents, three taxonomy tree files are necessary in addition to the reference fasta files: – taxonomy tree files:
nodes.dmp from the NCBI taxonomy dump. Links taxonomy IDs to their parents names.
names.dmp from the NCBI taxonomy dump. Links taxonomy IDs to their scientific name.
seqid2taxid.map is a tab-separated file that maps sequence IDs to taxonomy IDs.
• When using the provided scripts to download the genomes, these files are automatically downloaded or generated. When using custom taxonomy or sequence files, please refer to the manual section TODO to learn more about their format. Here is the link to the manual https://ccb.jhu.edu/software/centrifuge/manual.shtml
• command flags:
-o specifies the folder to which the files are downloaded. taxonomy Argument indicates the database to download. The options are refseq, genbank, contaminants or taxonomy.
# Takes ~20 seconds
source activate centrifuge
centrifuge-download -o /home/$USER/centrifuge/taxonomy taxonomy• Download complete fungal genomes from RefSeq with the centrifuge–download commands. Do this exercise for this tutorial on a reduced data set.
• command flags:
-a Only download genomes with the specified assembly level. Default: ’Complete Genome’. Use ’Any’ for any assembly level.
-m Mask low-complexity regions using dustmasker. This is part of the blast program that was installed in the metagenomics environment.
-P Number of threads or processes to use when downloading.
-d What domain to download. One or more of bacteria, viral, archaea, fungi, protozoa, invertebrate, plant, vertebrate_ mammalian, vertebrate_other (comma separated). refseq Tells the program to use the refseq database.
# Takes about 1 minute
centrifuge-download -o library -a "Complete Genome" -m -P 4 -d "fungi" refseq > seqid2taxid.map• Do not do: This database was pre-built for you to use in the tutorial. Download complete archaea, bacteria, viral and fungal genomes from RefSeq with the centrifuge–download commands. This is what you should do to build a more comprehensive RefSeq database containing only complete genomes.
• At the time of download, there were 289 Archea, 13,287 bacterial, 8,583 viral, and 10 fungal genomes.
centrifuge-download -o library -a "Complete Genome" -m -P 30 -d "archaea,bacteria,viral,fungi" refseq \
> seqid2taxid.map• Download the RefSeq chromosome level human genome reference with centrifuge-download commands.
• Command flags are the same as the previous step:
-t Only download the specified taxonomy IDs, comma separated. Default: any. 9606 is the taxonomy id for Human.
-c Only download genomes in the specified refseq category. Default: any.
Appends the human seqid2taxid. map to the previously generated fungi seqid2taxid.map file.
# Took ~14 minutes during testing
centrifuge-download -o library -a "Chromosome" -m -P 4 -d "vertebrate_mammalian" \
-t 9606 -c "reference genome" refseq >> seqid2taxid.map• *Optional. Customize your database by adding EuPathDB release 28 reference genomes. This step was done for you. The EuPathDB data comes from this publication. – Here is the link to the publication https://doi.org/10.1371/journal.pcbi.1006277 . Here is the webpage for the data download https://ccb.jhu.edu/data/eupathDB/
# Make new directory called eupath in the library directory. Something like this
mkdir -p /home/$USER/centrifuge/library/eupath
# Move into the /home/$USER/centrifuge/library/eupath directory.
cd /home/$USER/centrifuge/library/eupath
# This wget command will download EuPathDB data from the following link.
wget http://ccb.jhu.edu/data/eupathDB/dl/eupathDB.tar.gz
# This may take a long time. Perhaps copy instead:
cp /home/elavelle/centrifuge/library/eupath/eupathDB.tar.gz ./
# Uncompress the eupathDB.tar.gz file into the eupath directory. You should see
# a directory named library
tar -zxvf eupathDB.tar.gz
# Move the fasta files from the /home/$USER/centrifuge/library/eupath/library directory
#to /home/$USER/centrifuge/library/eupath/ directory
mv /home/$USER/centrifuge/library/eupath/library/*.fna \
/home/$USER/centrifuge/library/eupath/
# Extract and append the contig/scaffold and taxonomy id information from the prelim_map.txt that was
# downloaded with eupathDB.tar.gz file to the seqid2taxid.map file
awk -F'\t' '{print $2,$3}' /home/$USER/centrifuge/library/eupath/library/\
prelim_map.txt | awk -F'|' '{print $1,$3}' | awk -F' ' '{print $1"\t"$2}' \
>> /home/$USER/centrifuge/seqid2taxid.map• Concatenate the reference genome fasta files that were downloaded with the centrifuge-download commands. We are using a for loop to do this; otherwise, we would have to type every file name on the command-line that we want to concatenatee. g., cat fasta_1.fna fasta_2.fna …. > > all_fasta_files.fna . For practice, we will concatenate the fungi reference genome fasta files and the human fasta file.
10.6 Database building
Build the Centrifuge indexes using the centrifuge-build command. This has been done for you already. It took 14 hrs and ~ 410 GBs of RAM using 30 threads to build this database index on logrus. And, this was just using the “Complete Genome” Refseq genomes for archea, bacteria, viruses, fungi, human, and EuPathDB. I attempted to build a Centrifuge index using “All” RefSeq genomes for archea, bacteria, viruses, fungi, human plus the EuPAthDB but ran out of memory on a high memory server and Centrifuge was using 890 GBs of RAM before I killed the program. So, unless you have high-capacity compute you may not be able to build the indexes on your system. But, the Centrifuge developers have provided various premade indexes that you can download here, ftp://ftp.ccb.jhu.edu/pub/infphilo/centrifuge/data/p+h+v.tar.gz, with wget.
• Do this exercise for this tutorial. Build the Centrifuge database indexes with the centrifuge-build command. This is to practice building an index.
• command flags:
-p The number of processes/threads to use for creating the index.
–conversion-table The seqid2taxid.map file that you created with the centrifuge-download commands.
–taxonomy-tree The nodes.dmp file that was downloaded with the first centrifuge-download command.
–name-table The nodes.dmp file that was downloaded with the first centrifuge-download command.
# Each run will use about 24 GB of RAM.
# It took 39 minutes to run during testing.
centrifuge-build -p 4 --conversion-table /home/$USER/centrifuge/seqid2taxid.map --taxonomy-tree \
/home/$USER/centrifuge/taxonomy/nodes.dmp --name-table \
/home/$USER/centrifuge/taxonomy/names.dmp \
/home/$USER/centrifuge/library/fungi_HumanFixed.fna \
/home/$USER/centrifuge/fungi_Human_indeces10.7 Classification
In this portion of the tutorial we will use the pre-built indexes that contain complete RefSeq genomes for archea, bacteria, viruses and fungi. The human chromosome level reference genome was included and we also added the EuPathDB database as well. Each file will use about 36 GB of RAM, so not every one can run samples at the same time.
• Classify reads with Centrifuge.
• command flags:
-x Path to the indexes
– Index location /home/metag/centrifuge/complete_genomes/Arc_Bac_Vir_Hum_Eupath_v2
-1 Read 1 fastq file
-2 Read 2 fastq file
-t Print wall-clock time taken by search phase. This is optional.
-p Number of alignment threads to launch
–met-file Send metrics to file at
–met-stderr Send metrics to stderr
-S Output file name
# Make a directory to store your results:
mkdir -p /home/$USER/centrifuge/centrifuge_results
# Run Centrifuge. It will take 10 - 20 minutes per sample to run.
centrifuge -x /data/centrifuge/complete_genomes/Arc_Bac_Vir_Hum_Eupath_v2 \
-1 /data/centrifuge/ERR2271042_1.fastq \
-2 /data/centrifuge/ERR2271042_2.fastq -t \
-p 4 --met-file /home/$USER/centrifuge/centrifuge_results/ERR2271042_meta.txt --met-stderr \
-S /home/$USER/centrifuge/centrifuge_results/ERR2271042_cent.out
# If you have many samples to analyze, use a for loop to run centrifuge on each sample sequentially.
# Use a for loop to store the names of the fastq files in a variable (like a list)
# named fastq using the basename command
fastq=`for i in data/centrifuge/*_1.fastq; do basename -s _1.fastq $i;done`
# You can view the contents of the fastq variable with the echo command
echo $fastq
# Use a second for loop to read through the variable list and run centrifuge on each file in the list
# sequentially.
for j in $fastq; do centrifuge -x /data/centrifuge/complete_genomes/Arc_Bac_Vir_Hum_Eupath_v2 \
-1 /data/centrifuge/${j}_1.fastq \
-2 /data/centrifuge/${j}_2.fastq -t -p 4 \
--met-file /home/$USER/centrifuge/centrifuge_results/${j}_meta.txt --met-stderr \
-S /home/$USER/centrifuge/centrifuge_results/${j}_cent.out; done10.8 Convert output to Kraken-style reports
• Create a Kraken style report from the Centrifuge output using the centrifuge-kreport command. This command used about 17 GB of memory and took about 1 minute per file to complete during testing.
• command flags:
-x Path to the indexes.
# Make a directory to store your output files:
mkdir -p /home/$USER/centrifuge/centrifuge_krn_results
centrifuge-kreport -x /data/centrifuge/complete_genomes/Arc_Bac_Vir_Hum_Eupath_v2 \
/home/$USER/centrifuge/centrifuge_results/ERR2271042_cent.out \
> /home/$USER/centrifuge/centrifuge_krn_results/ERR2271042_cent.out.krn• Use a for loop if you have many samples to analyze.
# Use a for loop to store the names of the fastq files in a variable (like a list)
# named output using the basename command
output=`for i in /home/metag/centrifuge/centrifuge_results/*_cent.out; do basename -s _cent.out $i;done`
# You can view the contents of the fastq variable with the echo command
echo $output
# Use a second for loop to read through the variable list and run centrifuge on each file in the list
# sequentially.
for j in $output; do centrifuge-kreport \
-x /data/centrifuge/complete_genomes/Arc_Bac_Vir_Hum_Eupath_v2 \
/home/$USER/centrifuge/centrifuge_results/${j}_cent.out > \
/home/$USER/centrifuge/centrifuge_krn_results/${j}.krn; done10.9 Parse Metadata
• Use awk to extract columns 2, 10, and 18 from the metadata file located at /home/metag/centrifuge/data/ PRJEB23207_metadata.txt . Then, pipe the output to grep and exclude samples that were sequenced with the “Ion Torrent PGM” platform
# Copy the /data/centrifuge/PRJEB23207_metadata.txt to your working directory
cp /data/centrifuge/PRJEB23207_metadata.txt \
/home/$USER/centrifuge
# Use awk and grep to parse the metadata file
# -F is the field separator flag. In this case the metadata file is tab-separated '\t'.
# -i case-insensitive, -v exclude the search string, i.e., exclude lines that contain "ion"
awk -F'\t' '{print $2,$10,$18}' PRJEB23207_metadata.txt | grep -i -v "ion"10.10 Visualize the Kraken Reports with Pavian
• First, download all *.krn files to your computer using secure copy (scp) with unix, linux, or MobaXterm terminals.
• This assumes that you have installed R on your computer.
# Run R by opening a terminal
R
# Or click on your R application from R-studio or R-console.
# Install Pavian if you haven't done so:
if (!require(remotes)) { install.packages("remotes") }
remotes::install_github("fbreitwieser/pavian")
# Start Pavian in your browser from R
pavian::runApp(port=5000)• A web browser should pop up. If not, Pavian can be accessed by entering this url into your browser. http://127.0.0.1:5000
• Drag the .krn files to the “Browse” bar under the “Upload files” tab.
• Click on the “Results Overview” tab on the left side of the screen.
• Click on the “Sample” tab on the left side of the screen. You will notice an interactive Sankey chart that displays different classified taxa. You can click and drag nodes to reorder the nodes if you want. You can select different samples by clicking on the down arrow next to the “Select sample” header. Select sample ERR2270960 or ERR2270961 since we know that these two samples are likely infected with STEC.
• From the “Sample” page, click on the “Table” tab. You should see an interactive, filterable and searchable table that summarizes the results of Centrifuge. For example, type “Escherichia coli” into the “Search:” box. A histogram will appear that displays the numbers of reads across all samples that were classified as “Escherichia coli”. Sort the table by the most abundant taxa by clicking on the up arrow next to “TaxonReads”.
• Can you can you identify the strain(s) of STEC that samples ERR2270960 and ERR2270961 likely contain?
10.11 References
Gigliucci F, von Meijenfeldt FAB, Knijn A, Michelacci V, Scavia G, Minelli F, Dutilh BE, Ahmad HM, Raangs GC, Friedrich AW, Rossen JWA, Morabito S. Metagenomic Characterization of the Human Intestinal Microbiota in Fecal Samples from STECInfected Patients. Front Cell Infect Microbiol. 2018 Feb 6;8:25. doi: 10.3389/fcimb.2018.00025. eCollection 2018. PubMed PMID: 29468143; PubMed Central PMCID: PMC5808120.
Kim D, Song L, Breitwieser FP, Salzberg SL. Centrifuge: rapid and sensitive classification of metagenomic sequences. Genome Res. 2016 Dec;26(12):1721-1729. Epub 2016 Oct 17. PubMed PMID: 27852649; PubMed Central PMCID: PMC5131823.
Lu J, Salzberg SL. Removing contaminants from databases of draft genomes. PLoS Comput Biol. 2018 Jun 25;14(6):e1006277. doi: 10.1371/journal.pcbi.1006277. eCollection 2018 Jun. PubMed PMID: 29939994; PubMed Central PMCID: PMC6034898.