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Chlamydiae are obligate intracellular organisms, that is, 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.


• Chlamydia trachomatis causes eye, respiratory and genital tract infections. C. trachomatis is the most common cause of sexually transmitted disease in the United States.
• Chlamydia pneumoniae causes atypical pneumonia.
• C. psittaci causes psittacosis.

C. 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.

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 replicate cycle different from that of all other bacteria.
• The cycle begins when the extracellular, metabolically inert, spore like elementary body enters the cell and reorganizes into a larger, metabolically active reticulate body.
• The latter undergoes repeated binary fission to form daughter elementary bodies, which are released from the cell. Within cells, the site of replication appears as an inclusion body, which can be stained and visualized microscopically.
• These inclusions are useful in the identification of these organisms in the clinical laboratory.
• All chlamydiae share a group-specific lipopolysaccharide antigen, which is detected by complement fixation tests.
• They also possess species-specific and immune type-specific antigens (proteins), which are detected by immunofluorescence. C. psittaci and C. pneumoniae each have 1 immune type, whereas C. trachomatis has at least 15.

Transmission & Epidemiology

• C. trachomatis infects only humans and is usually transmitted by close personal contact, e.g., sexually or by passage through the birth canal.
• Individuals with asymptomatic genital tract infections are an important reservoir of infection for others.
• In trachoma, C. trachomatis is transmitted by finger-to-eye or fomite-to-eye contact.
• Sexually transmitted disease caused by C. trachomatis occurs worldwide, but trachoma is most frequently found in developing countries in dry, hot regions such as northern Africa.
• Trachoma is a leading cause of blindness in those countries.
• Patients with a sexually transmitted disease are co infected with both C. trachomatis and Neisseria gonorrhoeae in approximately 10–30% of cases.
• C. pneumoniae infects only humans and is transmitted from person to person by aerosol.
• C. psittaci infects birds and many mammals. Humans are infected primarily by inhaling organisms in dry bird feces.

Pathogenesis & Clinical Findings

• Chlamydiae infect primarily epithelial cells of the mucous membranes or the lungs.
• They rarely cause invasive, disseminated infections.
• C. psittaci infects the lungs primarily.
• The infection may be asymptomatic (detected only by a rising antibody titer) or may produce high fever and pneumonia.
• Human psittacosis is not generally communicable.
• C. pneumoniae causes upper and lower respiratory tract infections, especially bronchitis and pneumonia, in young adults.
• C. trachomatis exists in more than 15 immune types (A–L).
• Types A, B, and C cause trachoma, a chronic conjunctivitis endemic in Africa and Asia.
• Trachoma may recur over many years and may lead to blindness but causes no systemic illness.
• Types D–K cause genital tract infections, which are occasionally transmitted to the eyes or the respiratory tract.
• In men, it is a common cause of non gonococcal urethritis, which may progress to epididymitis, prostatitis, or proctitis.
• In women, cervicitis develops and may progress to salpingitis and pelvic inflammatory disease (PID).
• Repeated episodes of salpingitis or PID can result in infertility or ectopic pregnancy.
• Infants born to infected mothers often develop mucopurulent conjunctivitis (neonatal inclusion conjunctivitis) 7–12 days after delivery, and some develop chlamydial pneumonitis 2–12 weeks after birth.
• Chlamydial conjunctivitis also occurs in adults as a result of the transfer of organisms from the genitals to the eye.
• Patients with genital tract infections caused by C. trachomatis have a high incidence of Reiter's syndrome; which is an autoimmune disease caused by antibodies formed against C. trachomatis cross-reacting with antigens on the cells of the urethra, joints, and uveal tract and presented with urethritis, arthritis, and uveitis.
• C. trachomatis L1–L3 immune types cause lymphogranuloma venereum; a sexually transmitted disease with lesions on genitalia and in lymph nodes.
• Infection by C. trachomatis leads to formation of antibodies and cell-mediated reactions but not to resistance to reinfection or elimination of organisms.

Laboratory Diagnosis

• Chlamydiae form cytoplasmic inclusions, which can be seen with special stains (e.g.,

Giemsa stain) or by immunofluorescence.
• The Gram stain is not useful.
• In exudates, the organism can be identified within epithelial cells by fluorescent antibody staining or hybridization with a DNA probe.
• Chlamydial antigens can also be detected in exudates or urine by ELISA (enzyme-linked immuno sorbent assay).
• A polymerase chain reaction (PCR)-based test using the patient's urine can also be used to diagnose chlamydial sexually transmitted disease.
• Tests not involving culture are now more commonly used than culture-based tests.
• Chlamydiae can be grown in cell cultures treated with cycloheximide, which inhibits host cell but not chlamydial protein synthesis, thereby enhancing chlamydial replication.
• In culture, C. trachomatis forms inclusions containing glycogen, whereas C. psittaci and C. pneumoniae form inclusions that do not contain glycogen.
• The glycogen-filled inclusions are visualized by staining with iodine.
• Exudates from the eyes, respiratory tract, or genital tract give positive cultures in about half of cases.
• Serologic tests are used to diagnose infections by C. psittaci and C. pneumoniae but are rarely helpful in diagnosing disease caused by C. trachomatis because the frequency of infection is so high that many people already have antibodies.


• All chlamydiae are susceptible to tetracyclines, such as doxycyline, and macrolides, such as erythromycin and azithromycin.
• The drug of choice for C. trachomatis sexually transmitted diseases is azithromycin.
• Because the rate of co infection with gonococci and C. trachomatis is high, any patient with a diagnosis of gonorrhea should also be treated for C. trachomatis with azithromycin.
• The drug of choice for neonatal inclusion conjunctivitis is erythromycin. The drug of choice for C. psittaci and C. pneumoniae infections and for lymphogranuloma venereum is a tetracycline, such as doxycycline.


• There is no vaccine against any chlamydial disease.
• The best preventive measure against C. trachomatis sexually transmitted diseases is to limit transmission by prompt treatment of both the patient and the sexual partners, including persons who are asymptomatic.
• Sexual contacts should be traced, and those who had contact within 60 days should be treated.
• Several types of sexually transmitted diseases are often present simultaneously. Thus, diagnosis of one requires a search for other causative agents.
• Erythromycin given to newborn infants of infected mothers can prevent inclusion conjunctivitis and pneumonitis caused by C. trachomatis.
• Psittacosis in humans is controlled by restricting the importation of psittacine birds, destroying sick birds, and adding tetracycline to bird feed.
• Domestic flocks of turkeys and ducks are tested for the presence of C. psittaci.