<unk>  is a gram-negative bacterium that can replicate only within a host cell. Over the course of the<unk>  life cycle, the bacteria take on two distinct forms. Elementary bodies are 200 to 400 nanometers across, and are surrounded by a rigid cell wall that allows them to survive outside of a host cell. This form can initiate a new infection if it comes into contact with a susceptible host cell. Reticulate bodies are 600 to 1500 nanometers across, and are found only within host cells. Neither form is motile. The<unk>  genome is substantially smaller than that of many other bacteria at approximately 1.04 megabases, encoding approximately 900 genes. Several important metabolic functions are not encoded in the<unk>  genome, and instead, are likely scavenged from the host cell. In addition to the chromosome that contains most of the genome, nearly all<unk>  strains carry a 7.5 kilobase plasmid that contains 8 genes. The role of this plasmid is unknown, though strains without the plasmid have been isolated, suggesting it is not required for survival of the bacterium.
Chlamydia trachomatis
Neisseria gonorrhoeae
Trichomonas vaginalis
Mycoplasma genitalium
Haemophilus ducreyi
Treponema pallidum
Neisseria mucosa
Helicobacter suis