The Chlamydiota (synonym Chlamydiae) are a bacterial phylum and class whose members are remarkably diverse, including pathogens of humans and animals, symbionts of ubiquitous protozoa,[4] and marine sediment forms not yet well understood.

[5] Historically it was believed that all Chlamydiota had a peptidoglycan-free cell wall, but studies in the 2010s demonstrated a detectable presence of peptidoglycan, as well as other important proteins.

They are dependent on replication inside the host cells; thus, some species are termed obligate intracellular pathogens and others are symbionts of ubiquitous protozoa.

The term chlamydia (a cloak) appeared in the literature in 1945, although other names continued to be used, including Bedsonia, Miyagawanella, ornithosis-, TRIC-, and PLT-agents.

[17] This proposal is supported by the observation of two distinct phylogenetic clades that warrant taxonomic ranks above the family level.

[29][30] However, CSIs and CSPs have been found specifically for Chlamydophila species, supporting their distinctness from Chlamydia, perhaps warranting additional consideration of two separate groupings within the family.

The Chlamydiota form a unique bacterial evolutionary group that separated from other bacteria about a billion years ago, and can be distinguished by the presence of several CSIs and CSPs.

However, phylogeny and shared presence of CSIs in proteins that are lineage-specific indicate that the Verrucomicrobiota are the closest free-living relatives of these parasitic organisms.

In one extreme example, two genes encoding histone-like H1 proteins of eukaryotic origin have been found in the prokaryotic genome of C. trachomatis, an obligate intracellular pathogen.

Simkaniaceae Waddliaceae Parachlamydiaceae Chlamydiaceae "Parilichlamydiaceae" Simkaniaceae "Rhabdochlamydiaceae" "Ca.s Rubidus" Waddliaceae "Criblamydiaceae" Parachlamydiaceae Chlamydiaceae The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)[44] and National Center for Biotechnology Information (NCBI)[45]