Abstract
This article provides an overview of the current knowledge on chlamydiae, which are intracellular bacteria belonging to the Chlamydiaceae family. Whole-genome sequencing leads to great increases in the available data about Chlamydia spp. Recently, novel chlamydial taxons in various hosts living in different environments have been recognised. New species and taxons with Candidatus status have been recorded mainly in birds and reptiles. Chlamydia gallinacea is an emerging infectious agent in poultry with indirectly confirmed zoonotic potential. Recently, a new group of avian C. abortus strains with worldwide distribution in various wild bird families has been described. The definition of C. abortus species became outdated with the discovery of these strains and has been amended. It now includes two subgroups, mammalian and avian, the latter including all isolates hitherto referred to as atypical C. psittaci or C. psittaci/C. abortus intermediates.