Chlamydia pneumoniae : studies on an emerging pathogen

The Chlamydiae are characterized as bacteria because of the composition of their cell wall and their growth by binary division. However, they are obligate intracellular bacterial parasites of eukaryotic cells that have a unique replication cycle similar to Rickettsia. Chlamydial infection of host cells is initiated by the elementary body (? = 300-400 nm) , a stable structure specifically adapted to survive the extracellular phase during transit between cells. According to the knowledge, the infectious elementary body enters the cell via endocytosis (90,118). Intracellular parasites have evolved several ways to avoid being killed by lysosomal enzymes (139). However, the mechanism by which subsequent fagosome/lysosome fusion is avoided, is not completely understood. After endocytosis of the elementary body, it will reorganize into a non-infectious, metabolic active reticulate body (? = 800 - 1000 nm), which is responsible for intracellular replication through binary fission. At both stages of development the chlamydial cell is surrounded by an envelope similar to that of gram-negative bacteria. The envelope consists of two trilalninar lnenlbranes, an outer lnelnbrane and an inner, cytoplasmic membrane (24). The fissions occur in the original phagosome that enlarges during this process until it resembles a vacuole. This vacuole is referred to as an inclusion body or in short, an inclusion. During replication, the reticulate body obtains adenosine triphosphate, amino acids and sugars fi'om the host cell (7). Each reticulate body will reorganize into one or more elementary bodies (204). After 48-72 hrs, release of infective elementary bodies will take place due to host cell lysis (Figure 1). The order Chlamydiales has one family, Chlamydiaceae, and one genus, Chlamydia. There are now four recognized species, Chlamydia trachomatis, Chlamydia psittaci, Chlamydia pneumoniae and Chlamydia pecol'1lm. Of these, the most recently described C.peCOl'llm infects sheep and cattle, and has so far not been associated with human disease (58,59). At this moment, 19 different C.tl'achomatis serovars or serotypes are known, i.e. distinguished from each other by polyclonal antibodies that were formed in mice after they have been infected with a single type Chlamydia (201).

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