The Chlamydiaceae are 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-(2→8)-αKdo-(2→4)-αKdo (previously called the genus-specific epitope). Chlamydiaceae ribosomalRNA genes all have at least 90% DNA sequence identity. Chlamydiaceae species have varying inclusion morphology, varying extrachromosomal plasmid content, and varying sulfadiazine resistance.
C. trachomatis is the cause of an infection commonly transmitted sexually (often referred as just "Chlamydia") and also is the cause of trachoma, an infectious eye disease, spread by eye, nose, and throat secretions.
Six species belong to Chlamydophila: C. pneumoniae (often also called Chlamydia pneumoniae), C. pecorum, C. psittaci, C. abortus, C. caviae, and C. felis. Chlamydophila species do not produce detectable glycogen and have one ribosomal operon. These species are naturally found living in a variety of animals, where they are known to cause, in some cases, extreme sexual discomfort and mental pain.