metagenome-taxonomic-profiling

Skill: metagenome-taxonomic-profiling

Use When

• The user wants to determine which organisms are present in a metagenomic sample.
• The user needs taxonomic abundance estimates at species, genus, or phylum level.
• The user wants to compare Kraken2 vs MetaPhlAn profiling approaches.
• The user needs Bracken re-estimation for more accurate species-level abundance from Kraken2 output.

Inputs

• Required:
  • Host-depleted FASTQ file(s) (.fastq, .fq, .fastq.gz, .fq.gz).
• Optional:
  • --profiler STR — Profiler to use: kraken2 or metaphlan (default: kraken2).
  • --db PATH — Kraken2 or MetaPhlAn database path.
  • --bracken-db PATH — Bracken database path (for Bracken re-estimation).
  • --bracken-len N — Read length for Bracken (e.g., 100, 150, 250).
  • --level CHAR — Taxonomic level for Bracken: S (species), G (genus), P (phylum), etc. (default: S).
  • --threads N — Number of threads (default: 4).
  • --outdir DIR — Output directory (default: taxonomic_profiling_results).
  • --confidence FLOAT — Kraken2 confidence threshold (default: 0.0).

Workflow

1. If Kraken2: run kraken2 with --report to generate a Kraken-style taxonomic report.
2. If Bracken: run bracken on the Kraken2 report for abundance re-estimation at the specified taxonomic level.
3. If MetaPhlAn: run metaphlan with --input_type fastq to produce a merged abundance table.
4. Parse reports: extract top N taxa, relative abundances, and compute diversity metrics (Shannon, Simpson).
5. Report: classification rate, top taxa, total species detected, alpha diversity estimates.

Output Contract

• Kraken2 report — Standard Kraken2 taxonomic report (<outdir>/<sample>_kraken2_report.txt).
• Bracken abundance table — Re-estimated abundances at the specified level (<outdir>/<sample>_bracken.txt), if Bracken is used.
• MetaPhlAn profile — Relative abundance table (<outdir>/<sample>_metaphlan_profile.txt), if MetaPhlAn is used.
• Classification rate — Percentage of reads assigned to a taxon.
• Top taxa summary — Top 20 taxa by relative abundance.
• Diversity metrics — Shannon and Simpson alpha diversity indices.

Limits

• Kraken2 standard database is approximately 70 GB and requires sufficient RAM to load into memory.
• MetaPhlAn 4 uses marker genes; it has a smaller database and higher species-level precision but may miss novel taxa without markers.
• Bracken requires a pre-built Bracken database matching the Kraken2 database and the read length used in sequencing.
• Kraken2 and MetaPhlAn must be installed and available on $PATH.
• Classification rates vary significantly by database completeness and sample type.
• For low-biomass samples, consider using a higher Kraken2 confidence threshold to reduce false positives.
• Common failure cases:
  • Kraken2 database too large to fit in available RAM, causing out-of-memory crash.
  • Bracken database read length not matching actual sequencing read length.
  • MetaPhlAn marker database version incompatible with the installed MetaPhlAn version.