Ena database

ENA Database — European Nucleotide Archive Programmatic Access

Overview

The European Nucleotide Archive (ENA) is EMBL-EBI's comprehensive nucleotide sequence database, encompassing raw sequencing reads, genome assemblies, annotated sequences, and associated metadata. It mirrors and extends INSDC data (GenBank, DDBJ). All access is via REST APIs with no authentication required.

When to Use

• Searching for sequencing studies, samples, or experiments by organism, project, or keyword
• Downloading raw FASTQ/BAM files for reanalysis of public sequencing datasets
• Retrieving genome assemblies with quality statistics (N50, contig count, genome size)
• Fetching nucleotide sequences in FASTA or EMBL flat-file format by accession
• Exploring taxonomic lineage and finding organisms by partial name
• Cross-referencing ENA records with external databases (ArrayExpress, UniProt, PDB)
• Building bulk download lists for large-scale sequencing projects
• For multi-database Python queries (ENA + UniProt + KEGG), prefer bioservices instead
• For NCBI-specific queries (PubMed literature, GenBank records), use pubmed-database or Biopython Entrez

Prerequisites

pip install requests

API constraints:

• Rate limit: 50 requests per second across all ENA APIs
• No authentication required
• Large result sets: use pagination (limit + offset) or streaming (limit=0 for TSV download)
• Portal API base: https://www.ebi.ac.uk/ena/portal/api
• Browser API base: https://www.ebi.ac.uk/ena/browser/api
• Taxonomy API base: https://www.ebi.ac.uk/ena/taxonomy/rest
• Cross-ref API base: https://www.ebi.ac.uk/ena/xref/rest

Quick Start

import requests
import time

BASE_PORTAL = "https://www.ebi.ac.uk/ena/portal/api"
BASE_BROWSER = "https://www.ebi.ac.uk/ena/browser/api"
BASE_TAXONOMY = "https://www.ebi.ac.uk/ena/taxonomy/rest"
BASE_XREF = "https://www.ebi.ac.uk/ena/xref/rest"

def ena_query(endpoint, params=None, base=BASE_PORTAL):
    """Reusable ENA API caller with rate-limit compliance."""
    resp = requests.get(f"{base}/{endpoint}", params=params)
    resp.raise_for_status()
    time.sleep(0.02)  # 50 req/sec limit
    return resp

# Search for human RNA-seq studies
resp = ena_query("search", params={
    "result": "study",
    "query": 'tax_tree(9606)',   # `library_strategy` is a `read_run`/`read_experiment` field, not a `study` field
    "fields": "study_accession,study_title",
    "format": "json",
    "limit": 3,
})
studies = resp.json()
for s in studies:
    print(f"{s['study_accession']}: {s['study_title'][:60]}")
# PRJEB12345: Transcriptome analysis of human liver tissue...

Core API

Module 1: Portal API Search

The Portal API provides advanced metadata search across all ENA data types with boolean query syntax, field selection, and pagination.

# Search read runs for a specific study
resp = ena_query("search", params={
    "result": "read_run",
    "query": 'study_accession="PRJEB1787"',
    "fields": "run_accession,sample_accession,instrument_model,read_count,base_count",
    "format": "json",
    "limit": 5,
})
runs = resp.json()
for r in runs:
    print(f"{r['run_accession']} — {r.get('instrument_model', 'N/A')}, "
          f"{int(r.get('read_count', 0)):,} reads")
# ERR123456 — Illumina HiSeq 2000, 45,231,890 reads

# Count total results without fetching data
count_resp = ena_query("count", params={
    "result": "read_run",
    "query": 'study_accession="PRJEB1787"',
})
print(f"Total runs: {count_resp.text.strip()}")
# Total runs: 142

Module 2: Browser API Retrieval

Fetch individual records by accession in multiple formats: XML, FASTA, EMBL flat-file, or plain text.

# Retrieve XML metadata for a study
resp = ena_query("xml/PRJEB1787", base=BASE_BROWSER)
print(resp.text[:300])
# <?xml version="1.0" encoding="UTF-8"?><PROJECT_SET>...

# Retrieve FASTA sequence for a coding sequence
resp = ena_query("fasta/M10051.1", base=BASE_BROWSER)
print(resp.text[:200])
# >ENA|M10051|M10051.1 Human insulin mRNA, complete cds.
# AGCCCTCCAGGACAGGCTGCAT...

# Retrieve EMBL flat-file format
resp = ena_query("embl/M10051.1", base=BASE_BROWSER)
print(resp.text[:300])
# ID   M10051; SV 1; linear; mRNA; STD; HUM; 786 BP.
# ...

Module 3: File Reports and Downloads

Get download URLs for FASTQ, submitted, and analysis files. File reports return FTP and Aspera paths.

# Get FASTQ file URLs for specific runs
resp = ena_query("filereport", params={
    "accession": "ERR000589",
    "result": "read_run",
    "fields": "run_accession,fastq_ftp,fastq_bytes,fastq_md5",
    "format": "json",
})
files = resp.json()
for f in files:
    ftp_urls = f.get("fastq_ftp", "").split(";")
    sizes = f.get("fastq_bytes", "").split(";")
    for url, size in zip(ftp_urls, sizes):
        if url:
            print(f"ftp://{url}  ({int(size)/1e6:.1f} MB)")
# ftp://ftp.sra.ebi.ac.uk/vol1/fastq/ERR000/ERR000589/ERR000589_1.fastq.gz  (234.5 MB)
# ftp://ftp.sra.ebi.ac.uk/vol1/fastq/ERR000/ERR000589/ERR000589_2.fastq.gz  (241.2 MB)

Module 4: Taxonomy Queries

Look up organisms by taxonomy ID, scientific name, or partial name match.

# Lookup by taxonomy ID
resp = ena_query("tax-id/9606", base=BASE_TAXONOMY)
tax = resp.json()
print(f"{tax['scientificName']} (taxId: {tax['taxId']}, rank: {tax['rank']})")
# Homo sapiens (taxId: 9606, rank: species)
print(f"Lineage: {tax['lineage'][:80]}...")

# Search by scientific name — endpoint returns a list (one entry per matching taxon)
resp = ena_query("scientific-name/Arabidopsis thaliana", base=BASE_TAXONOMY)
matches = resp.json()
result = matches[0] if isinstance(matches, list) else matches
print(f"Tax ID: {result['taxId']}, Common: {result.get('commonName', 'N/A')}")
# Tax ID: 3702, Common: thale cress

# Suggest organisms by partial name
resp = ena_query("suggest-for-search/salmo", base=BASE_TAXONOMY)
suggestions = resp.json()
for s in suggestions[:3]:
    print(f"  {s['scientificName']} (taxId: {s['taxId']})")
# Salmo salar (taxId: 8030)
# Salmo trutta (taxId: 8032)
# Salmonella enterica (taxId: 28901)

Module 5: Cross-Reference Service

Find links between ENA records and external databases (ArrayExpress, UniProt, PDB, etc.).

# Find cross-references for an ENA accession
resp = ena_query("json/search", base=BASE_XREF, params={
    "accession": "M10051",
})
xrefs = resp.json()
for x in xrefs[:5]:
    print(f"  {x['Source']} → {x['Source Primary Accession']} "
          f"({x.get('Source Description', '')[:50]})")
# UniProt → P01308 (Insulin precursor)
# PDB → 1A7F (Crystal structure of human insulin)

# Search cross-references by external database
resp = ena_query("json/search", base=BASE_XREF, params={
    "source": "UniProt",
    "accession": "P01308",
})
xrefs = resp.json()
for x in xrefs[:3]:
    print(f"  ENA: {x['Target Primary Accession']} — {x.get('Target Description', '')[:60]}")

Module 6: CRAM Reference Registry

Retrieve reference sequences used in CRAM files by MD5 or SHA1 checksum. Essential for CRAM decompression.

# Look up reference by MD5 checksum
md5 = "aef131c3b4b05d8e2b3f907faba5af9b"  # example
try:
    resp = ena_query(
        f"cram/md5/{md5}",
        base="https://www.ebi.ac.uk/ena/cram"
    )
    print(f"Reference found: {len(resp.content)} bytes")
except requests.HTTPError as e:
    if e.response.status_code == 404:
        print("Reference not found — check MD5 checksum")
    else:
        raise

Key Concepts

ENA Data Hierarchy

Level	Accession Prefix	Description	Contains
Study	PRJEB/ERP	Research project	Samples, Experiments
Sample	ERS/SAMEA	Biological sample	Metadata, taxonomy
Experiment	ERX	Library/sequencing setup	Runs
Run	ERR	Sequencing run	Raw read files (FASTQ)
Analysis	ERZ	Derived analysis	Assemblies, alignments
Assembly	GCA	Genome assembly	Contigs, scaffolds
Sequence	Accession.version	Annotated sequence	Features, coding seqs

Query Syntax Operators

Operator	Example	Description
Equality	instrument_model="Illumina NovaSeq 6000"	Exact match
Wildcard	study_title="*melanoma*"	Partial match
Range	base_count>=1000000	Numeric comparison
Taxonomy tree	tax_tree(9606)	Taxon and all descendants
Exact taxon	tax_eq(9606)	Exact taxon only
Date range	first_public>=2023-01-01	Date filtering
Boolean	AND, OR, NOT	Combine conditions
Grouping	(A OR B) AND C	Parenthetical grouping

Result Types

Result Type	Description	Key Fields
study	Research projects	study_accession, study_title, center_name
sample	Biological samples	sample_accession, tax_id, scientific_name
read_run	Sequencing runs	run_accession, read_count, base_count, fastq_ftp
read_experiment	Experiments	experiment_accession, library_strategy, instrument_model
analysis	Derived analyses	analysis_accession, analysis_type
assembly	Genome assemblies	assembly_accession, assembly_level, genome_representation
sequence	Annotated sequences	accession, sequence_length, mol_type
wgs_set	WGS scaffold sets	set_accession, set_size
tsa_set	Transcriptome assemblies	set_accession, set_size
coding	Coding sequences	accession, gene, product
noncoding	Non-coding features	accession, description
taxon	Taxonomy entries	tax_id, scientific_name, lineage

Discoverable Fields

Use the returnFields endpoint to discover available fields for any result type:

resp = ena_query("returnFields", params={"result": "read_run"})
fields = resp.text.strip().split("\n")
print(f"Available fields for read_run: {len(fields)}")
print(fields[:10])
# ['accession', 'altitude', 'assembly_quality', 'assembly_software', ...]

Common Workflows

Workflow 1: Study Exploration Pipeline

Search for a study, list its samples, then retrieve run metadata.

import json

# Step 1: Find studies by organism — `study_title="*…*"` wildcards no longer match;
# use `tax_tree()` against the species tax ID (SARS-CoV-2 = 2697049).
resp = ena_query("search", params={
    "result": "study",
    "query": 'tax_tree(2697049) AND first_public>=2023-01-01',
    "fields": "study_accession,study_title,center_name",
    "format": "json",
    "limit": 3,
})
studies = resp.json()
study_acc = studies[0]["study_accession"]
print(f"Selected: {study_acc} — {studies[0]['study_title'][:60]}")

# Step 2: List samples in the study
resp = ena_query("search", params={
    "result": "sample",
    "query": f'study_accession="{study_acc}"',
    "fields": "sample_accession,scientific_name,collection_date",
    "format": "json",
    "limit": 5,
})
samples = resp.json()
print(f"Found {len(samples)} samples (showing first 5)")
for s in samples:
    print(f"  {s['sample_accession']} — {s.get('scientific_name', 'N/A')}")

# Step 3: Get run metadata for each sample
for s in samples[:2]:
    resp = ena_query("search", params={
        "result": "read_run",
        "query": f'sample_accession="{s["sample_accession"]}"',
        "fields": "run_accession,instrument_model,read_count,library_strategy",
        "format": "json",
    })
    runs = resp.json()
    for r in runs:
        print(f"  {r['run_accession']}: {r.get('library_strategy','N/A')}, "
              f"{int(r.get('read_count',0)):,} reads")
    time.sleep(0.02)

Workflow 2: Bulk FASTQ Download URL Collection

Search for runs matching criteria and collect download URLs.

# Step 1: Search for Illumina RNA-Seq runs from a specific organism
resp = ena_query("search", params={
    "result": "read_run",
    "query": ('tax_tree(10090) AND library_strategy="RNA-Seq" '
              'AND instrument_platform="ILLUMINA" AND read_count>=10000000'),
    "fields": "run_accession,study_accession,read_count",
    "format": "json",
    "limit": 10,
})
runs = resp.json()
print(f"Found {len(runs)} runs meeting criteria")

# Step 2: Get file reports with download URLs
download_list = []
for run in runs[:5]:
    acc = run["run_accession"]
    resp = ena_query("filereport", params={
        "accession": acc,
        "result": "read_run",
        "fields": "run_accession,fastq_ftp,fastq_bytes,fastq_md5",
        "format": "json",
    })
    for f in resp.json():
        urls = f.get("fastq_ftp", "").split(";")
        md5s = f.get("fastq_md5", "").split(";")
        for url, md5 in zip(urls, md5s):
            if url:
                download_list.append({"url": f"ftp://{url}", "md5": md5, "run": acc})
    time.sleep(0.02)

print(f"\nDownload list: {len(download_list)} files")
for d in download_list[:4]:
    print(f"  {d['run']}: {d['url'].split('/')[-1]}")

Workflow 3: Taxonomic Assembly Exploration

Find organisms, search their assemblies, and check quality statistics.

# Step 1: Resolve organism by exact scientific name (returns a list, take the first match).
# `suggest-for-search` only returns prefix matches and may not include the species you want.
resp = ena_query("scientific-name/Drosophila melanogaster", base=BASE_TAXONOMY)
matches = resp.json()
target_tax = matches[0]["taxId"]
print(f"Selected: {matches[0]['scientificName']} (taxId={target_tax})")

# Step 2: Search assemblies for this organism. `n50` is no longer a valid `assembly`
# field — use `base_count` and `program` for what's available.
resp = ena_query("search", params={
    "result": "assembly",
    "query": f'tax_eq({target_tax}) AND assembly_level="chromosome"',
    "fields": ("assembly_accession,assembly_name,assembly_level,"
               "genome_representation,base_count"),
    "format": "json",
    "limit": 5,
})
assemblies = resp.json()
for a in assemblies:
    size = int(a.get("base_count", 0))
    print(f"  {a['assembly_accession']}: {a.get('assembly_name','N/A')}, "
          f"Size={size/1e6:.1f} Mb")
# GCA_000001215.4: Release 6 plus ISO1 MT, Size=143.7 Mb

Key Parameters

Endpoint	Parameter	Default	Description
search	result	(required)	Result type: study, sample, read_run, assembly, etc.
search	query	(required)	Boolean query string with field operators
search	fields	all	Comma-separated field names to return
search	format	tsv	Output format: json, tsv, xml
search	limit	100000	Max results (0 = unlimited streaming)
search	offset	0	Skip first N results (pagination)
search	sortFields	—	Field(s) to sort by
filereport	accession	(required)	Study, sample, or run accession
filereport	result	(required)	read_run, analysis, etc.
xml/	accession path	(required)	Any ENA accession (Browser API)
fasta/	accession path	(required)	Sequence accession (Browser API)

Best Practices

1. Use tax_tree() over tax_eq() for organism queries — it includes subspecies and strains automatically
2. Request only needed fields — reduces response size and server load significantly
3. Prefer JSON format for programmatic access; TSV for large bulk exports (lower overhead)
4. Use limit=0 for streaming large result sets directly to file, avoiding memory issues
5. Check fastq_ftp and submitted_ftp — some runs have submitted files but no processed FASTQ
6. Verify downloads with MD5 — file reports include fastq_md5 for integrity checking
7. Anti-pattern: Do not fetch all fields then filter client-side — use query syntax server-side

Common Recipes

Recipe: Assembly Quality Filtering

# Find chromosome-level, full-representation assemblies.
# `n50` is no longer a valid `assembly` field — filter by base_count instead.
resp = ena_query("search", params={
    "result": "assembly",
    "query": ('tax_tree(7742) AND assembly_level="chromosome" '
              'AND genome_representation="full"'),
    "fields": "assembly_accession,scientific_name,base_count,assembly_level",
    "format": "json",
    "limit": 10,
})
for a in resp.json():
    size = int(a.get("base_count", 0))
    if size > 100_000_000:   # >100 Mb (proxy for "well-assembled")
        print(f"{a['assembly_accession']}: {a.get('scientific_name','?')}, Size={size/1e6:.0f} Mb")

Recipe: Cross-Database Linking

# Find UniProt/PDB cross-references for an ENA sequence
resp = ena_query("json/search", base=BASE_XREF, params={
    "accession": "M10051",
})
xrefs = resp.json()
by_source = {}
for x in xrefs:
    src = x.get("Source", "unknown")
    by_source.setdefault(src, []).append(x["Source Primary Accession"])
for src, accs in by_source.items():
    print(f"  {src}: {', '.join(accs[:5])}")
# UniProt: P01308
# PDB: 1A7F, 1AI0, 1BEN

Recipe: Retry Session with Exponential Backoff

from requests.adapters import HTTPAdapter
from urllib3.util.retry import Retry

def ena_session():
    """Create a requests session with retry logic for ENA APIs."""
    session = requests.Session()
    retry = Retry(
        total=3, backoff_factor=1.0,
        status_forcelist=[429, 500, 502, 503, 504],
    )
    session.mount("https://", HTTPAdapter(max_retries=retry))
    return session

session = ena_session()
resp = session.get(f"{BASE_PORTAL}/search", params={
    "result": "study",
    "query": 'tax_tree(9606)',
    "fields": "study_accession",
    "format": "json",
    "limit": 5,
})
print(f"Status: {resp.status_code}, results: {len(resp.json())}")

Troubleshooting

Problem	Cause	Solution
400 Bad Request on search	Invalid query syntax or unknown field name	Use returnFields endpoint to verify field names; check query operator syntax
400 with result param	Invalid result type	Check result types table above; common: read_run not run
Empty results for known data	Wrong taxonomy operator	Use tax_tree() (includes descendants) not tax_eq() (exact only)
fastq_ftp field is empty	Submitted files not processed to FASTQ	Check submitted_ftp field instead; some datasets only have BAM/CRAM
429 Too Many Requests	Exceeded 50 req/sec rate limit	Add time.sleep(0.02) between requests; use retry session with backoff
Timeout on large queries	Result set too large for single request	Use limit + offset pagination, or limit=0 with streaming to file
XML parsing errors	Malformed XML for some records	Use JSON format instead (format=json) when available
Wrong sequence version	Accession without version suffix	Always use versioned accessions (e.g., M10051.1 not M10051) for Browser API
CRAM reference not found	MD5 checksum mismatch or non-INSDC reference	Verify MD5; check if reference is from a custom genome (not in registry)

Bundled Resources

This skill is self-contained. The original entry had a separate references/api_reference.md (490 lines) covering all 6 API endpoints in detail. That content has been fully consolidated inline:

• Portal API (search, count, returnFields, filereport) — Core API Modules 1 and 3, Key Parameters table
• Browser API (XML, FASTA, EMBL retrieval) — Core API Module 2
• Taxonomy REST API (tax-id, scientific-name, suggest-for-search) — Core API Module 4
• Cross-Reference Service (json/search) — Core API Module 5
• CRAM Reference Registry (md5/sha1 lookup) — Core API Module 6
• Rate limiting and error handling — Prerequisites, Troubleshooting, Recipe 3 (retry session)
• Query syntax and result types — Key Concepts section (3 tables)
• Pagination and bulk download — Key Parameters, Best Practices, Workflow 2
• Omitted: detailed EMBL format field-by-field breakdown (rarely needed programmatically); Aspera download command examples (tool-specific, not requests-based)

Related Skills

• bioservices-multi-database — unified Python interface covering ENA via bioservices; prefer for multi-database workflows
• pubmed-database — PubMed literature search via NCBI E-utilities
• pysam-genomic-files — downstream processing of FASTQ/BAM/CRAM files retrieved from ENA
• biopython-molecular-biology — NCBI Entrez access and sequence parsing (GenBank/FASTA)
• ncbi-blast (planned) — BLAST sequence similarity search

References

• ENA Portal API docs: https://www.ebi.ac.uk/ena/portal/api/doc
• ENA Browser API docs: https://www.ebi.ac.uk/ena/browser/api/doc
• ENA Taxonomy REST API: https://www.ebi.ac.uk/ena/taxonomy/rest/
• ENA Cross-Reference Service: https://www.ebi.ac.uk/ena/xref/rest/
• ENA data model guide: https://ena-docs.readthedocs.io/en/latest/submit/general-guide/data-model.html
• INSDC standards: https://www.insdc.org/