The bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l order Chlamydiales includes only obligately intracellular bacteria that have a chlamydia-like developmental cycle of replication and at least 80% 16S rRNA or 23S rRNA gene sequence identity with other members of Chlamydiales. Chlamydiales live in animals, insects, and protozoa. The order Chlamydiales belongs to the class Chlamydiae, phylum Chlamydiae, domain Bacteria.

At present, Chlamydiae

Chlamydiae

Chlamydiae is a bacterial phylum and class whose members are obligate intracellular pathogens. Many Chlamydiae coexist in an asymptomatic state within specific hosts, and it is widely believed that these hosts provide a natural reservoir for these species....

 and Chlamydiales are essentially synonymous.

Currently, the order Chlamydiales includes the families Chlamydiaceae

Chlamydiaceae

Chlamydiaceae is a family of bacteria that belongs to the Phylum Chlamydiae, Order Chlamydiales. All Chlamydiaceae species are Gram-negative and express the family-specific lipopolysaccharide epitope αKdo--αKdo--αKdo . Chlamydiaceae ribosomal RNA genes all have at least 90% DNA sequence identity...

, Simkaniaceae

Simkaniaceae

Simkaniaceae is a family of bacteria in the order Chlamydiales, class Chlamydiae, phylum Chlamydiae, domain Bacteria. Species in this family have a chlamydia-like cycle of replication and their ribosomal RNA genes are 80–90% identical to ribosomal genes in the Chlamydiaceae...

, and Waddliaceae

Waddliaceae

Waddlia is a genus of bacteria in its own family, Waddliaceae. Species in this genus have a Chlamydia-like cycle of replication and their ribosomal RNA genes are 80–90% identical to ribosomal genes in the Chlamydiaceae....

, which have Gram-negative extracellular infectious bodies (EBs), and Parachlamydiaceae

Parachlamydiaceae

Parachlamydiaceae is a family of bacteria in the order Chlamydiales. Species in this family have a Chlamydia–like cycle of replication and their ribosomal RNA genes are 80–90% identical to ribosomal genes in the Chlamydiaceae. The Parachlamydiaceae naturally infect amoebae and can be grown in...

, which has variable Gram staining of EBs. The family Rhabdochlamydiaceae

Rhabdochlamydiaceae

Candidatus Rhabdochlamydia is a genus of intracellular bacteria; a separate family, Candidatus Rhabdochlamydiaceae, has been proposed for this genus. Candidatus Rhabdochlamydia species have not been cultured in vitro and have not been deposited in culture collections.Two Rhabdochlamydia species...

has been proposed.