Garrity and Holt (2001): The phylum Chlamydiae is defined on the basis of 16S rDNA sequence data as a separate evolutionary lineage within the Bacteria. Ludwig and Klenk regard the Chlamydiae as a sister group of the Verrucomicrobia, but caution that the relationship is tentative. More recently, Everett et al. (1999) proposed an emendation of the Chlamydiales, subdividing the order into four families: Chlamydiaceae, Parachlamydiaceae, Simkaniaceae, and Waddliaceae. We have incorporated this scheme into the current version of the outline and have added the class "Chlamydiae" and the phylum Chlamydiae to complete the hierarchy. In PCA plots, the Chlamydiaceae, Parachlamydiaceae, and Waddliaceae form a tightly clustered group that is clearly separated from the Planctomycetes and Verrucomicrobia, with which the Chlamydiae have often been grouped. The Simkaniaceae are clearly separate from the remaining members of the phylum, an observation that is consistent with the published trees (Everett et al., 1999), suggesting a possible misplacement. All members of the phylum are nonmotile, obligately parasitic, coccoid bacteria that multiply within membrane-bound vacuoles in the cytoplasm of cells of mammalian and avian origin. Gram-negative or Gram-variable (Parachlamydia). Multiplication occurs by means of a complex life cycle. Pathogenic. Cell walls do not contain muramic acid or only a trace. (BMDB9 Group 9).
Oren and Garrity (2021): The properties of the taxon are as described by Garrity and Holt (2001). Correction of the effectively published synonym: Chlamydiae (sic) Garrity and Holt 2001. Type genus: Chlamydia Jones et al. 1945 (Approved Lists 1980).