Taxonomic Note: A Place for DNA-DNA Reassociation and 16S rRNA Sequence Analysis in the Present Species Definition in Bacteriology

This taxonomic note addresses how to define bacterial species using molecular methods, specifically DNA-DNA reassociation and 16S rRNA sequence analysis. The authors argue that while species cannot be recognized as naturally isolated genetic groups, a pragmatic polyphasic approach is necessary for classification stability. DNA-DNA reassociation at approximately 70% similarity remains the gold standard for species delineation because it correlates well with phenotypic and genomic characteristics. However, rapid 16S rRNA sequencing has become widespread, making its role in species definition unclear. The authors demonstrate that 16S rRNA sequence analysis has limited resolution power for closely related organisms, with species sharing >97% sequence identity often showing variable DNA reassociation values (as low as 25%). They conclude that 16S rRNA analysis is valuable for determining when DNA reassociation studies should be performed—particularly when organisms share <97% sequence homology—but cannot replace DNA hybridization for precise species delineation at the strain level. The relationship between these two methods is non-linear, with each superior in different taxonomic ranges.

Key findings

• DNA-DNA reassociation at ≥70% similarity with thermal stability differences ≤5°C remains the superior method for bacterial species delineation and strain-level classification
• 16S rRNA sequence analysis has limited resolution power for closely related organisms; species with 97-99.8% rRNA sequence identity can show DNA reassociation values ranging from 25-100%
• Organisms with <97% 16S rRNA sequence homology are unlikely to exceed 60-70% DNA similarity, making rRNA analysis useful for determining when DNA reassociation studies are necessary
• The correlation between 16S rRNA homology and DNA-DNA reassociation is non-linear, with each method superior in different taxonomic ranges from domains down to species level