Accessing data from NCBI with EDirect

Using NCBI's EDirect

Unix

NCBI is definitely pretty awesome. But sometimes it can be a little tricky to figure out how to download the data we want – particularly when it’s a lot of things and we want and/or need to do it at the command-line rather than at the site. There are some convenient tools available that may help in some situations depending on our needs. For searching by taxonomy and downloading genomes (assemblies), @kaiblin and some others have put together a great tool called ncbi-genome-download, and they have one for downloading individual sequences by accession called ncbi-acc-download. I also put together a small program specifically for downloading assemblies by accession when making GToTree, and I found it useful in general so it’s also now part of my Bioinf Tools as bit-dl-ncbi-assemblies.

But there have been times when I needed more than these could offer, which required me spending a decent amount of time getting used to using NCBI’s EDirect tools. EDirect is a set of tools NCBI provides to enable accessing the vast amount of information stored at NCBI from the command line. Learning to use it at all was painful at times for me – and still is when I’m trying to figure out new stuff with it. But like a lot of things, once I had a little familiarity with it, I started to appreciate how useful and powerful it can be.


Why use EDirect?

Being able to access data and info from NCBI at the command line can allow us to: automate and document things well (we can give the exact command used to retrieve information and the date it was executed, rather than “pulled from NCBI”); download directly to a server rather than our local computer; pull more specific information than we can on the site; and more 🙂

There is a lot of info on this at the main NCBI EDirect page, and some more here. Those are where I’ve learned from (along with a lot of trial and error). But I still have trouble finding examples when I need them (and I always need them when using this). And I often end up digging through several of my old log files to find things, so…

This page holds some of the ways I’ve used EDirect, both to serve as a handy archive for myself, and to hopefully help others 🙂 It won’t be as comprehensive as most other things on this site, as it’s extremely expansive and it’s still nowhere near intuitive for me 🤷‍♂️ But these examples may do what is needed, and if not they at least might provide good starting points for building the code that will do what is needed.

NOTE: This stuff can look messy, mostly because it is; it scares me too. This page assumes you already have some familiarity with working at the command line. If you don’t yet, then definitely consider running through the Unix crash course first 🙂





Using NCBI's EDirect

For a while, this was also kind of a huge pain to install on some systems. But thanks to the gloriousness of conda, that’s no longer the case 🙂


Conda install

conda install -y -c conda-forge -c bioconda -c defaults entrez-direct



Accessing genome assemblies and info

Getting all Alteromonas assembly accessions (e.g. to input into GToTree like the example here!)

esearch -db assembly -query '"Alteromonas"[Organism] AND latest[filter] AND \
        (all[filter] NOT anomalous[filter] AND all[filter] NOT "derived from \
        surveillance project"[filter])' | esummary | xtract -pattern \
        DocumentSummary -element AssemblyAccession > Alteromonas-assembly-accs.txt

NOTE: We can build the search string at the NCBI website and copy and paste it from there (there’s a little “Search Details” box at the right side of the search page that adds in things as we modify our search on the site).


Getting all RefSeq Bacteria assembly accessions, taxids, assembly status, number of contigs, L50, N50, and total assembly length (took ~5 minutes to get 166,566 records as accessed on 1-Sep-2019)

esearch -db assembly -query '"Bacteria"[Organism] AND "latest refseq"[filter] AND \
        (all[filter] NOT anomalous[filter] AND all[filter] NOT "derived from \
        surveillance project"[filter])' | esummary | xtract -pattern DocumentSummary \
        -def "NA" -element AssemblyAccession,Taxid,assembly-status -block Stat \
        -if Stat@category -equals contig_count -or Stat@category -equals contig_l50 \
        -or Stat@category -equals contig_n50 -or Stat@category -equals total_length \
        -sep ":" -def "NA" -element Stat@category,Stat \
        > All-bacteria-refseq-complete-assembly-info.tsv

Downloading genomes by accession

I typically do this part using bit-dl-ncbi-assemblies from my Bioinf Tools after getting the accessions like in the examples above. Here’s what that would look like following the Alteromonas example above to download those 160 assemblies (here in fasta format; took ~1 minute):

bit-dl-ncbi-assemblies -w Alteromonas-assembly-accs.txt -f fasta -j 10

But here’s an example using EDirect to pull the sequence data for a RefSeq accession:

esearch -db assembly -query GCF_006538345.1 | elink -target nucleotide -name \
        assembly_nuccore_refseq | efetch -format fasta > GCF_006538345.1.fa

And one for a GenBank accession:

esearch -db assembly -query GCA_006538345.1 | elink -target nucleotide -name \
        assembly_nuccore_insdc | efetch -format fasta > GCA_006538345.1.fa

Note the change in the -name parameter between those two. “assembly_nuccore_insdc” is for GenBank, while “assembly_nuccore_refseq” is for RefSeq. Also note that I have no idea what the underlying infrastructure is here, and have to trial-and-error things whenever I’m trying to find something for the first time. Two places to look are with the einfo -dbs command and at this site here.




Protein annotations from assembly accession

esearch -db assembly -query GCA_006538345.1 | elink -target nuccore -name \
        assembly_nuccore_insdc | elink -target protein | efetch -format gb \
        -mode xml | xtract -pattern GBSeq -element GBSeq_accession-version \
        -block GBQualifier -if GBQualifier_name -equals product \
        -element GBQualifier_value > GCA_006538345.1-annotations.tsv



Accessing proteins

Sequences and accessions

Getting amino acid sequences based on protein-name text search

esearch -db protein -query '"nosZ"[Protein name]' | efetch -format fasta > nosZ.faa

Getting only unique sequences from the Identical Protein Groups database

esearch -db IPG -query '"nosZ"[Protein name]' | efetch -format fasta > nosZ-IPG.faa

Getting nucleotide coding sequences for proteins based on protein-name text search

esearch -db protein -query '"nosZ"[Protein name]' | efetch -format fasta_cds_na > nosZ.fa

Getting protein accessions based on protein-name text search

esearch -db protein -query '"nosZ"[Protein name]' | esummary | xtract -pattern \
        DocumentSummary -element AccessionVersion > nosZ-accs.txt

Getting a single protein sequence by accesssion

efetch -db protein -format fasta -id ABA21534.1

Getting many protein sequences by accession
We can’t provide an input file to the efetch command, but we can to epost first. Assuming we have our target accessions in a single-column file, this can be done like so:

echo -e "ABA21534.1\nWP_013322114.1\nWP_015207051.1" > accs.txt

epost -input accs.txt -db protein | efetch -format fasta

NOTE: If doing this with tens of thousands of targets, we need to split things up. There is a section at the end of this page covering this.


Getting the nucleotide coding sequence for a protein from the protein accession

efetch -db protein -format fasta_cds_na -id ABA21534.1

Getting protein sequences from an assembly (genome) accession

esearch -db assembly -query GCA_006538345.1 | elink -target nuccore -name \
        assembly_nuccore_insdc | elink -target protein | efetch -format fasta \
        > GCA_006538345.1.faa



GIs from accessions

NCBI has unique GI IDs for things in addition to their accessions. I needed GIs specifically once in order to block them from a command-line BLAST search as it can take them to block, but not a list of accessions.

Getting a single GI from a protein accession

esearch -db protein -query AEE52072.1 | esummary | xtract -pattern DocumentSummary \
        -element Gi

Getting many GIs from protein accessions
We can use epost similar to above:

epost -input accs.txt -db protein | esummary | xtract -pattern DocumentSummary \
      -element Gi

IMPORTANT

epost will not return things in the same order you input them. If you need to preserve the order, you should consider returning the accession as well, e.g.:

epost -input accs.txt -db protein | esummary | xtract -pattern DocumentSummary \
      -element AccessionVersion,Gi

And then we can sort things based on the accession to make the input and outputs retain order.




Protein annotations from accessions

Getting a single protein annotation from its accession

efetch -db protein -format gb -mode xml -id ABA21534.1 | xtract -pattern GBSeq \
       -element GBSeq_accession-version -block GBQualifier -if GBQualifier_name \
       -equals product -element GBQualifier_value

Getting many protein annotations from their accessions

epost -input accs.txt -db protein | efetch -format gb -mode xml | xtract -pattern \
      GBSeq -element GBSeq_accession-version -block GBQualifier -if GBQualifier_name \
      -equals product -element GBQualifier_value



Breaking up lots of queries

The other thing we have to address is that the Entrez site notes that we shouldn’t submit more queries than the site can handle for the database we are targeting. This doesn’t matter when we’re doing a single search that is grabbing lots of things (like the example above to get all RefSeq bacterial accessions), but rather if we are submitting lots of things that need to be queried individually.

I needed to do this at some point, but I can’t remember exactly for what. So this example will be a little contrived, but will show how I did it. Let’s first grab all the protein accessions from an Alteromonas with assembly accession GCA_006538345.1:

esearch -db assembly -query GCA_006538345.1 | elink -target nuccore -name \
        assembly_nuccore_insdc | elink -target protein | esummary | xtract -pattern \
        DocumentSummary -element AccessionVersion > GCA_006538345.1-prot-accs.txt

This is 4,135 accessions. And here’s a nice little bash function to help do this that comes from the NCBI EDirect page:

JoinIntoGroupsOf() {
  xargs -n "$@" echo |
  sed 's/ /,/g'
}

This will take chunks of our accessions and provide them separated by commas to our input. It’s also a good idea to provide our email address when we are submitting a lot of things. That way if we cause a problem on their end, they can contact us to tell us and we won’t keep breaking the system until we get banned 😬

cat GCA_006538345.1-prot-accs.txt | JoinIntoGroupsOf 200 | xargs -n 1 sh -c \
    'efetch -email "MyEmail@gmail.com" -db protein -id $0 -format fasta' \
    > GCA_006538345.1.faa

That was submitting chunks of 200 at a time and took about 20 seconds for me. Looking at this page here of the Entrez database links we can see that nuccore_protein says maximum items processed of 5,000. So I think we can set this value as high as that (which in this case is higher than our total targets, but this is how we’d do it if we had more):

cat GCA_006538345.1-prot-accs.txt | JoinIntoGroupsOf 5000 | xargs -n 1 sh -c \
    'efetch -email "MyEmail@gmail.com" -db protein -id $0 -format fasta' \
    > GCA_006538345.1.faa

And that took 6 seconds.





Again, sorry this isn’t a comprehensive or a fundamental explanation of any of this. I’m just not at that level with this particular skillset. xtract on its own is a pretty expansive language to learn, on top of needing to know the structure of how NCBI links all of its stuff together. It’s a lot! I’d like to know it better someday, and then would love to put together a page for it 🤞

For now, hopefully these examples being stored here will help more than just me 🙂