Skip to Main Content


Chlamydiae are obligate intracellular bacteria (i.e., they can grow only within cells). They are the agents of common sexually transmitted diseases, such as urethritis and cervicitis, as well as other infections, such as pneumonia, psittacosis, trachoma, and lymphogranuloma venereum.

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 593.


Chlamydia trachomatis causes eye (conjunctivitis, trachoma), respiratory (pneumonia), and genital tract (urethritis, lymphogranuloma venereum) infections. Chlamydia trachomatis is the most common bacterial cause of sexually transmitted disease in the United States. Infection with C. trachomatis is also associated with Reiter’s syndrome, an autoimmune disease.

Chlamydia pneumoniae causes atypical pneumonia. Chlamydia psittaci causes psittacosis, also a disease characterized mainly by pneumonia (Table 25–1).

TABLE 25–1Chlamydiae of Medical Importance

Chlamydia pneumoniae and C. psittaci are sufficiently different molecularly from C. trachomatis that they have been reclassified into a new genus called Chlamydophila. Taxonomically, they are now Chlamydophila pneumoniae and Chlamydophila psittaci. However, from a medical perspective, they are still known as Chlamydia pneumoniae and Chlamydia psittaci, and those are the names that are used in this book.

Important Properties

Chlamydiae are obligate intracellular bacteria. They lack the ability to produce sufficient energy to grow independently and therefore can grow only inside host cells. They have a rigid cell wall but do not have a typical peptidoglycan layer. Their cell walls resemble those of gram-negative bacteria but lack muramic acid.

Chlamydiae have a replicative cycle different from that of all other bacteria. The cycle begins when the extracellular, metabolically inert, “sporelike” elementary body enters the cell and reorganizes into a larger, metabolically active reticulate body (Figure 25–1). The latter undergoes repeated cycles of binary fission to form daughter reticulate bodies, which then develop into elementary bodies, which are released from the cell. Within cells, the site of replication appears as an inclusion body in the cytoplasm, which can be stained and visualized microscopically (Figure 25–2). These inclusions are useful in the identification of these organisms in the clinical laboratory.


Life cycle of Chlamydia. The extracellular, inert elementary body enters an epithelial ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.