Chlamydia are obligate intracellular Gram-negative bacteria. They have a dimorphic growth cycle of elementary bodies (EB), which are electron-dense infectious structures and reticulate bodies (RB), which are non-infectious, intracellular forms. Elementary bodies attach to the cell to initiate cell infection. Once inside the cells they differentiate into RBs; RBs divide by binary fission and subsequently differentiate back to EBs to be released from the cell to initiate further infection.

Chlamydia belong to the family Chlamydiaceae, which has two genera:

1. Chlamydia – which has one species C. trachomatis. C. trachomatis is further subdivided into serovars.

1. Serovars A, B, Ba and C cause trachoma (a tropical eye infection).

2. Serovars D–K cause genital infection.

3. Lymphogranuloma venereum (LGV) 1, 2, 3 cause genital infection with inguinal lymphadenopathy in the tropics.

1. Chlamydophila – species in the genus are:

1. Ch. psittaci – natural infection in birds, both psittacine (parrots, budgerigars etc.) and other birds (e.g. pigeons). Human infection is acquired as a zoonosis from birds.

2. Ch. pneumoniae – is a human pathogen.

3. Ch. abortus – primarily infects sheep and causes abortion in pregnant ewes (hence the name), human infection is accidental from sheep.

4. Ch. caviae – causes infection in guinea pigs, but does not cause human infection.

Epidemiology

Both chlamydia and chlamydophila are prevalent worldwide. Trachoma is a disease of underdeveloped and developing countries, and the most important cause of blindness in these parts.