[2] All Chlamydiota are anaerobic bacteria with a biphasic developmental lifecycle that depends on obligately intracellular growth in eukaryotic host cells.

For example, at that time genus C. psittaci was distinguished from C. trachomatis by sulfadiazine resistance, although not all strains identified as C. psittaci at the time were resistant, and C. pneumoniae was classified by its appearance under electron microscopy (EM) and its ability to infect humans, although the EM appearance may differ from one research group to the next, and many of these species infected humans.

Supporting criteria such as antigen detection,[5] glycogen staining, host association, and EM morphology were also employed, depending on applicability and availability.

[6] This view was challenged three years later by newer whole genome analysis techniques leading to a proposal to "reunite the Chlamydiaceae into a single genus, Chlamydia".

[10][11][12][13] According to the authors of the 1999 study, the mean DNA–DNA reassociation difference distinguishing Chlamydophila from Chlamydia is 10.1%, an accepted value for genus separation.