A group of 30 chlamydial researchers objected to a proposed reclassification of the order Chlamydiales that would divide the genus Chlamydia into two separate genera and create additional species. The authors acknowledged that Chlamydia psittaci and Chlamydia trachomatis each contained heterogeneous organisms, but argued the proposed reclassification was premature and problematic. Their main concerns were that the new scheme relied almost exclusively on minor differences in 16S and 23S rRNA genes without identifying new biological markers for differentiation. Some proposed species contained too few isolates to validate stability. The authors noted that 16S rRNA, while useful for evolutionary studies, is inadequate for species determination. They argued that the 95% sequence identity threshold for genus separation was arbitrary and unwarranted, as differences between C. trachomatis, C. psittaci, and C. pneumoniae were only 5-6%. They contended that recent phylogenetic analyses showed these organisms form a coherent single genus. The authors emphasized that the organisms share unique, highly conserved biology best recognized when classified together, and warned that introducing new nomenclature based solely on sequence data would create confusion in medical and public health fields without providing practical benefits or nomenclatural stability.

Key findings

• The proposed reclassification relied almost exclusively on minor 16S and 23S rRNA sequence differences without identifying new biological markers for genus or species differentiation.
• The 95% sequence identity threshold used to justify dividing the genus was arbitrary and unsupported, as maximum differences between major Chlamydia species were only 5-6%.
• 16S rRNA alone is insufficient for speciation, and genus-specific genes would be more appropriate taxonomic criteria.
• Some proposed new species contained too few isolates to ensure the observed sequence differences would be maintained with additional strains.
• The single genus Chlamydia represented a taxonomically and phylogenetically coherent grouping with unique, shared biology, and renaming would create practical confusion in medical and public health applications.