Species specificity

Species Specificity

Ensure your primers amplify only your target organism and ignore background DNA.

Use Cases

• Pathogen Detection: Detect Salmonella in a Human sample.
• Environmental DNA (eDNA): Identify specific species in water/soil samples.
• GMO Testing: Distinguish transgenic sequences from host crops.

How It Works

PrimerLab uses BLAST+ to check primer candidates against two databases:

1. Target Database: The genome(s) of the organism you WANT to amplify.
2. Background Database: The genome(s) of organisms you do NOT want to amplify (e.g., Human, Mouse, Soil metagenome).

It calculates a Specificity Score:

• + Points: Perfect matches in Target Database.
• - Points: Significant matches in Background Database, especially at the 3’ end.

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Configuration

Add a specificity block to your config:

# specificity_config.yaml
specificity:
  # Positive control: Primers MUST bind here
  target_database: ./data/salmonella_genome.fasta
  
  # Negative control: Primers MUST NOT bind here
  background_database: ./data/human_genome.fasta
  
  # Thresholds
  min_specificity_score: 90
  check_3_end: true  # Critical for specificity

Running the Check

Integrated Workflow

Run this during primer design to filter out non-specific primers automatically:

primerlab run pcr --config specificity_config.yaml --species-check

Standalone Tool

Check existing primers:

primerlab species-check \
  --primers my_primers.json \
  --target-db salmonella.fasta \
  --background-db human.fasta

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Interpreting Results

The tool outputs a summary table:

Primer Pair	Target Hits	Background Hits	Specificity Score	Status
Pair 1	1	0	100	✅ PASS
Pair 2	1	5	45	❌ FAIL
Pair 3	1	1 (weak)	85	⚠️ WARN

• FAIL: Significant background amplification likely.
• WARN: Weak background binding; may work if annealing temperature is optimized.