Careful use of 16S rRNA gene sequence similarity values for the identification of species.

In order to evaluate the suitability of 16S rRNA nucleotide sequence similarity for the classification of new isolates at the species level, we systematically studied the pairwise identity values of this gene for 131 species with standing in nomenclature. Only one of the studied species, (0.76%), strictly respected the 95% and 98.65% threshold values currently recommended to determine the affiliation of bacterial isolates to an existing or new genus or species, respectively. All other species exhibited at least an identity value >98.65% and/or <95% with another species. Therefore, we suggest that interpretation of interspecies 16S rRNA identity values should be made cautiously when classifying a new mycobacterial isolate at the species level.