1. Chlamydia trachomatis
2. Chlamydia pneumoniae (Chlamydophila pneumoniae)
3. Chlamydia psittaci (Chlamydophila psittaci)

Lifecycle of Chalmydia

• Extracellular form
• Metabolically inert (does not divide)
• Dense, round, small (300 nm), infectious particle
• The outer membrane has extensive disulfide bond cross-linkages that confer stability for extracellular existence (resistant to harsh environmental conditions)

• Intracellular form
• Metabolically active (replicates by binary fission)
• Non-infectious particles
• Possess a fragile membrane lacking the extensive disulfide bonds characteristic of the EB

1. The infectious particle is the elementary body (EB). The EB attaches to and enters columnar epithelial cells lining the mucous membranes via endocytosis.
2. Once within an endosome, the EB inhibits phagosome-lysosome fusion and is not destroyed. It transforms into a Reticulate body (RB).
3. Once enough RBs have formed by binary fission, some transform back into EB.
4. The resulting inclusions may contain 100 - 500 progeny
5. The life cycle is completed when the host cell liberates the elementary body (EB), which can now infect more cells.

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