Chlamydiae are
Chlamydiae represent a phylum of obligate intracellular Gram-negative bacteria unable to synthesize ATP and many metabolites, historically termed energy parasites relying on host nucleotide transporters. They display unique biphasic developmental cycle: infectious small elementary body with condensed chromatin and disulfide crosslinked envelope resistant outside host, and replicative large reticulate body metabolically active within membrane-bound inclusion. Reduced genome size around one megabase reflects host dependence. Pathogens Chlamydia trachomatis, pneumoniae, and psittaci cause ocular, genital, respiratory infections, distinct from free-living environmental bacteria.
Ref: Jawetz Medical Microbiology 27th ed., Chlamydiae obligate intracellular elementary body; Tortora Microbiology Chapter 11 Chlamydia developmental cycle