ANI and Species Delineation
One-Sentence Definition
Average nucleotide identity (ANI) quantifies genome-wide sequence similarity between isolates; ~95–96% ANI is widely used as a pragmatic boundary for bacterial species delineation alongside phylogeny and GTDB Taxonomy.
Simple Explanation
Instead of arguing from one gene (16S), compare nearly whole genomes: if two bacteria share about 95%+ average DNA identity, they are usually called the same species.
Detailed Scientific Explanation
| Metric | Typical use |
|---|---|
| ANI (MUMmer / BLAST / sketching) | Species cutoffs (~95–96%) |
| AAI | More distant relationships |
| dDDH (digital DNA–DNA hybridization) | Formal taxonomic proposals |
| 16S rRNA identity | Quick but low resolution |
Tools: FastANI, OrthoANI, skani (fast sketches). Within-species outbreak work uses much finer methods (MLST and cgMLST, SNP distances), not ANI.
Caveats: species complexes (Acinetobacter baumannii–calcoaceticus, Enterobacter cloacae complex, Mycobacterium abscessus group) need careful interpretation; horizontal transfer can blur edges.
Mechanism
Align or sketch genome pairs → average identity over shared regions → compare to empirical species threshold → assign or split labels.
Clinical Importance
- Correct species name changes therapy and infection-control implications (mis-IDed non-fermenters, Mycobacterium complexes).
- MALDI databases can lag genomic taxonomy — ANI arbitrates disputes.
Research Importance
- Standard for proposing new species and cleaning public databases (Public Sequence Databases).
Diagnostic Relevance
- WGS reference labs use ANI/GTDB when biochemical/MALDI IDs conflict.
AMR Relevance
Species mis-assignment mis-maps expected intrinsic resistance (e.g., wrong Enterobacter member).
Related Methods
Related MOCs
Active Recall Questions
- Approximate ANI threshold for same species?
- Why is 16S insufficient for many pathogens?
- ANI vs cgMLST — which for outbreak vs species ID?