A new bacterial strain shows less than 95% 16S rRNA sequence identity with known species. What is the most reliable next step to determine whether it is a new species?
- ACheck the GC content
- BRun a BLAST search
- CObserve its morphology with an electron microscope
- DPerform average nucleotide identity (ANI) analysis
Show answer and explanation
Correct answer: (d) Perform average nucleotide identity (ANI) analysis
Explanation
16S rRNA identity below about 98.7% suggests that a bacterium belongs to a new species, but 16S rRNA is too uniform to settle it on its own.
Average nucleotide identity (ANI) compares the whole genome of the new strain with those of its nearest known species. An ANI below about 95–96% is now the accepted standard for a separate species; it corresponds to the older 70% DNA–DNA hybridisation rule.
- GC content varies too little and too unevenly to define a species.
- A BLAST search is a first screen, not proof.
- Morphology can mislead in bacteria, which often look alike.