Even though 16S rRNA gene has been widely used in the classification and identification of Bacteria and Archaea, it has a limitation; some species/subspecies share identical or very similar 16S rRNA sequences. Sequences derived from these taxa cannot be differentiated, therefore the strain cannot be identified correctly at the species level on the basis of 16S rRNA sequencing alone. A “taxonomic group” is defined as a group of taxa (species/subspecies) that cannot be differentiated solely by 16S rRNA sequences. A typical example is the case of Escherichia coli and Shigella spp., which show almost identical 16S rRNA sequences. It is safer to identify such 16S rRNA sequences as a member of a species group that contains very similar 16S rRNA sequences, rather than to potentially wrongly assign them as “E. coli.”
For instance, the Escherichia coli taxonomic group contains some species in Escherichia and Shigella, whose 16S sequences are so similar that they can not be differentiated. The name of a taxonomic group usually is usually the name of a species with the highest nomenclatural priority (meaning the oldest name). Visit the E. coli group page.
Edited by JC