Aetiology
The first isolates of Chlamydia pneumoniae were obtained serendipitously during trachoma studies in the 1960s. On the basis of inclusion morphology and staining characteristics in cell culture, C pneumoniae initially was considered a Chlamydophila psittaci strain, although there was no avian exposure. Subsequent analysis demonstrated that this organism was distinct from both C psittaci and Chlamydia trachomatis.[4] Spread of C pneumoniae within families and enclosed populations, such as military recruits and people in nursing homes, has been described.[5][6][7]
Pathophysiology
Chlamydia pneumoniae is an obligate intracellular pathogen that has established a unique niche within the host cell. It causes a variety of diseases in animal species at virtually all phylogenic levels. Each C pneumoniae is characterised by a unique developmental cycle with morphologically distinct infectious and reproductive forms.[4]
After infection, the infectious particle called the elementary body attaches to the host cell and is taken into the cell by endocytosis that does not depend on the microtubule system. Within the host cell, the elementary body remains within a membrane-lined phagosome. The phagosome does not fuse with the host cell lysosome. The elementary body then differentiates into the replicative form, the reticulate body that undergoes binary fission. After approximately 36 hours, reticulate bodies differentiate back into elementary bodies. At approximately 48 hours post-infection, release may occur by cytolysis or by a process of exocytosis or extrusion of the whole inclusion, with the host cell left intact. In vitro studies have shown that C pneumoniae may enter a persistent state after treatment with certain cytokines such as interferon gamma, treatment with antibiotics, or restriction of certain nutrients. While in the persistent state, metabolic activity is reduced.[4] The ability to cause prolonged, often subclinical infection is one of the major characteristics of this organism.
Culture of C pneumoniae requires isolation in tissue culture. Because of the long life-cycle, treatment of C pneumoniae infection requires multidose treatment regimens. However, persistence of the organism occurs in approximately 20% of patients after treatment of respiratory infection.
C pneumoniae has also been isolated from non-human species, including horses, koalas, bandicoots, reptiles and amphibians; although the role that these infections may play in human disease is unknown.[8] The mode of transmission remains uncertain but probably involves infected respiratory tract secretions. Acquisition of infection by droplet aerosol was described during a laboratory accident.[9]C pneumoniae can remain viable on countertops for 30 hours and can survive small-particle aerosolisation.[10]
Classification
Taxonomy
Order Chlamydiales
Family Chlamydiaceae
Genus Chlamydia
Species Chlamydia pneumoniae
Use of this content is subject to our disclaimer