The Chlamydia genus of bacteria consists of four unique species that cause a variety of disease including genital tract infections and respiratory tract infections.

The chlamydiae have a unique life cycle that is different from all other microorganisms. They possess both RNA and DNA, differentiating them from viruses, and have cell walls similar in structure to gram-negative bacteria. They are non-motile and form intracellular inclusions in the host cell that can be seen by light microscope.

Chlamyiae reproduce in the cytoplasm of host cells and cannot survive outside the cell due to their limited metabolic capacity. The chlamydiae use ATP produced by the host cell to fuel their metabolic reactions. The infectious and environmentally stable form of the bacteria is known as an elementary particle or body, while the metabolically active replicating form, known as the reticulate particle or body, is noninfectious and is reliant on the host cell for survival.

The developmental cycle involves a number of steps:

1. Attachment of the elementary particle or body (infectious form of the organism) to a receptor on the host cell surface
2. Entry intro the host cell through a process similar to endocytosis
3. Organism reorganization from an elementary body to the active and replicative form, the reticulate particle (body)
4. Morphologic change of reticulate body back to elementary body
5. Release of the infectious elementary bodies

There are four species of chlamydiae:

• Chlamydia trachomatis
• Chlamydia psittaci
• Chlamydia pneumoniae
• Chlamydia pecorum

Chlamydis trachomatis

Chlamydia trachomatis is a human pathogen and is the most common sexually transmitted infection (STI) in North America. Within the species, there are a number of different types of C. trachomatis: each type referred to as a serotype: serotypes A, B, Ba and C are associated with trachoma, a preventable form of blindness. Serotypes L1, L2 and L3 are associated with lymphogranuloma venereum (LGV), a sexually transmitted infection that results in inflammation of genital tract lymph nodes and/or proctitis. LGV is uncommon in North America. Serotypes D through K are associated with genital tract infection (urethritis, cervicitis, epididymitis, Reiter’s syndrome, proctitis, endometritis and salpingitis).

When genital tract infection is passed to the newborn during birth, inclusion conjunctivitis and chlamydial pneumonia may develop. And, in areas where trachoma is endemic, child to child transmission is largely responsible for chlamydia transmission. Trachoma is endemic in many developing countries. Poor hygiene and unsanitary conditions contribute to transmission, and flies can act as vectors in spreading infectious organisms found in eye discharge. Eye infection begins as mucopurulent conjunctivitis that progresses to the cornea and may result in blindness.

Chlamydia psittaci

C. psittaci is ubiquitous among species of birds and is common in domestic mammals. Infection in birds usually involves the gastrointestinal tract, and is shed in feces. Humans can become infected through exposure to infected birds (psittacosis). Cats can develop respiratory tract infection (feline pneumonitis) and in cattle, infection may result in abortion (bovine abortion).

Chlamydia pneumoniae

C. pneumoniae is a human pathogen that causes respiratory tract infection. The genetic make-up of C. pneumoniae varies greatly from the other three species of chlamydia. There is less than 10% similarity between C. pneumoniae and the other species of Chlamydia.

Pneumonia caused by C. pneumoniae is sometimes referred to as walking pneumonia. Serological studies suggest that by the age of 40, almost half of the population has been exposed to C. pneumoniae. Infection spreads more easily within households, schools, workplaces and other close living quarters through exposure to respiratory secretions.

Chlamydia pecorum

The pathogenic role, if any, of C. pecorum is unclear.

Read more about Chlamydia and other sexually tranmitted infections:

Uncomplicated gonorrhea

Cervicitis

Sexually transmitted infection

PID: Risk factors and prevention

STI and STD

Pelvic inflammatory disease

Trichomonas vaginalis

Bacterial vaginosis

Human papillomavirus

Sources:

Sexually Transmitted Diseases - Chlamydia (Centres for Disease Control and Prevention)

Schachter J. Biology of Chlamydia trachomatis (Chapter 28) in Sexually Transmitted Diseases, Third Edition. McGraw-Hill, New York 1999.

Schachter J and WE Stamm. Chlamydia (Chapter 55). In Manual of Clinical Microbiology, Sixth Edition. American Society of Microbiology Press, Washington, 1995.