Changes in the taxonomy of the order Chlamydiales, after its separation from the order Rickettsiales, were presented. These changes resulted in the recognition of the following families: Chlamydiaceae, Chlavichlamydiaceae, Criblamydiaceae, Parachlamydiaceae, Piscichlamydiaceae, Rhabdochlamydiaceae, Simkaniaceae, and Waddliaceae. Other described changes concerned particularly the family Chlamydiaceae. Its genus Chlamydia was divided into Chlamydia and Chlamydophila. However, in the following years, a revision to the single original genus was made, based upon phylogenetic analysis of 16S and 23S rRNA genes of the strains belonging to these two taxonomic units. The review also discusses other families outside the family Chlamydiaceae, which contain so-called Chlamydia-related or Chlamydia-like organisms. Members of each family share a 16S rDNA gene sequence similarity >90%. Furthermore, characterisation of the pathogenecity is presented, focusing especially on the representatives of the family Chlamydiaceae, which cause animal infections, and describing their zoonotic potential. Available data on this topic, connected with the representatives of other families, were mentioned.
