Using sourmash LCA to do taxonomic classification

The sourmash lca sub-commands do k-mer classification using an “lowest common ancestor” approach. See “Some discussion” below for links and details.

Quickstart

(These sourmash lca classify and sourmash lca summarize steps require about 4 GB of RAM when using the genbank database, as below.)

First, install sourmash 2.0a4 or later.

pip install -U https://github.com/dib-lab/sourmash/archive/master.zip

Next, download a genbank LCA database for k=31:

curl -L -o genbank-k31.lca.json.gz https://osf.io/zskb9/download

Download a random genome from genbank:

curl -L -o some-genome.fa.gz ftp://ftp.ncbi.nlm.nih.gov/genomes/all/GCF/000/178/875/GCF_000178875.2_ASM17887v2/GCF_000178875.2_ASM17887v2_genomic.fna.gz

Compute a signature for this genome:

sourmash compute -k 31 --scaled=1000 some-genome.fa.gz

Now, classify the signature with sourmash lca classify,

sourmash lca classify --db genbank-k31.lca.json.gz \
    --query some-genome.fa.gz.sig

and this will give you a taxonomic identification of your genome bin, classify using all of the genbank microbial genomes.

You can also summarize the taxonomic distribution of the content with lca summarize:

sourmash lca summarize --db genbank-k31.lca.json.gz \
    --query some-genome.fa.gz.sig

To apply this to your own genome(s), replace some-genome.fa.gz above with your own filename(s).

You can also specify multiple databases and multiple query signatures on the command line; separate them with --db or --query.

A longer tutorial

Install sourmash as above; see Appendix (below) for dependencies.

Let’s start by building your own LCA database, using your own taxonomy.

Building your own LCA database

(This is an abbreviated version of this blog post, updated to use the sourmash lca commands.)

Download some pre-computed signatures:

curl -L https://osf.io/bw8d7/download?version=1 -o delmont-subsample-sigs.tar.gz
tar xzf delmont-subsample-sigs.tar.gz

Next, grab the associated taxonomy spreadsheet

curl -O -L https://github.com/ctb/2017-sourmash-lca/raw/master/tara-delmont-SuppTable3.csv

Build a sourmash LCA database named delmont.lca.json:

sourmash lca index tara-delmont-SuppTable3.csv delmont.lca.json delmont-subsample-sigs/*.sig

Using the LCA database to classify signatures

We can now use delmont.lca.json to classify signatures with k-mers according to the database we just created. (Note, the database is completely self-contained at this point.)

Let’s classify a single signature:

sourmash lca classify --db delmont.lca.json \
    --query delmont-subsample-sigs/TARA_RED_MAG_00003.fa.gz.sig

and you should see:

loaded 1 databases for LCA use.
ksize=31 scaled=10000
outputting classifications to stdout
ID,status,superkingdom,phylum,class,order,family,genus,species
TARA_RED_MAG_00003,found,Bacteria,Proteobacteria,Gammaproteobacteria,,,,
classified 1 signatures total

You can classify a bunch of signatures and also specify an output location for the CSV:

sourmash lca classify --db delmont.lca.json \
    --query delmont-subsample-sigs/*.sig \
    -o out.csv

The lca classify command supports multiple databases as well as multiple queries; e.g. sourmash lca classify --db delmont.lca.json other.lca.json will classify based on the combination of taxonomies in the two databases.

Some discussion

Sourmash LCA is using k-mers to do taxonomic classification, using the “lowest common ancestor” approach (pioneered by Kraken, and described here), to identify each k-mer. From this it can either find a consensus taxonomy between all the k-mers (sourmash classify) or it can summarize the mixture of k-mers present in one or more signatures (sourmash summarize).

The sourmash lca index command can be used to prepare custom taxonomy databases; sourmash will happily ingest any taxonomy, whether or not it matches NCBI. See the spreadsheet from Delmont et al., 2017 for an example format.

Appendix: Installing sourmash from scratch

To install sourmash on an Ubuntu or Debian system, run:

sudo apt-get -y update && \
sudo apt-get install -y python3.5-dev python3.5-venv make \
    libc6-dev g++ zlib1g-dev

this installs Python 3.5.

Now, create a local software install and populate it with Jupyter and other dependencies:

python3.5 -m venv ~/py3
. ~/py3/bin/activate
pip install -U pip
pip install -U Cython
pip install -U jupyter jupyter_client ipython pandas matplotlib scipy scikit-learn khmer

Last but not least, install sourmash from the LCA branch:

pip install -U https://github.com/dib-lab/sourmash/archive/add/lca.zip

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