Chlamydiae are obligate intracellular pathogens that have established a unique niche within the host cell. Until recently, the order Chlamydiales contained one family, Chlamydiaceae, which contained one genus, Chlamydia, with four recognized species: Chlamydia trachomatis, Chlamydia psittaci, Chlamydia pneumoniae, and Chlamydia pecorum. The species most important in human disease are C. trachomatis and C. pneumoniae. C. trachomatis infection is the most prevalent sexually transmitted pathogen and infectious disease in the United States today (1). The Centers for Disease Control and Prevention estimates that the number of new C. trachomatis infections exceeds 4 million annually (1,2). C. pneumoniae is now recognized as an important respiratory pathogen, and C. psittaci is primarily a zoonosis.
Taxonomic analysis using the 16S and 23S ribosomal ribonucleic acid (RNA) genes have suggested that the order Chlamydiales contains at least four distinct groups at the family level, and that within the family Chlamydiaceae there are two distinct lineages (3). Under this analysis, the family Chlamydiaceae would be split into two genera: Chlamydia, which would contain C. trachomatis and two new species, Chlamydia muridarum and Chlamydia suis, and Chlamydophila, which would contain C. pecorum, C. pneumoniae, and C. psittaci and three new species split off from C. psittaci. The Chlamydia genus is also distinguished from species of the new genus Chlamydophila by the presence of the glycogenlike amorphous substance in the inclusions and susceptibility to sulfonamides.
Chlamydiae are characterized by a unique developmental cycle with morphologically distinct infectious and reproductive forms: elementary body (EB) and reticulate body (RB). Chlamydiae have a Gram-negative envelope without detectable peptidogly-can, although genomic analysis has revealed that both C. trachomatis and C. pneumoniae encode for proteins forming a nearly complete pathway for synthesis of peptidogylcan, including penicillin-binding proteins (3). This is the basis for the so-called chlamydial peptidoglycan paradox, as it has been known for decades that chlamydia development is sensitive to ^-lactam antibiotics.
Chlamydiae also share a group-specific lipopolysaccharide antigen and utilize host adenosine triphosphate for the synthesis of chlamydial protein. Although chlamydiae are auxotrophic for three of four nucleoside triphosphates, they do encode functional glucose-catabolizing enzymes, which can be used for generation of adenosine triphosphate (3). As with peptidoglycan synthesis, for some reason these genes are turned off. All chlamydiae also encode an abundant protein called the major outer membrane protein (MOMP or OmpA), which is surface exposed in C. trachomatis and C. psittaci but apparently not in C. pneumoniae. The major outer membrane protein is the major determinant of the serologic classification of C. trachomatis and C. psittaci isolates.
Following infection, the infectious EBs, which are 200-400 ^m in diameter, attach to the host cell by a process of electrostatic binding and are taken into the cell by endocytosis, which does not depend on the microtubule system. Within the host cell, the EB remains within a membrane-lined phagosome. The phagosome does not fuse with the host cell lysosome. The inclusion membrane is devoid of host cell markers, but lipid markers traffic to the inclusion, which suggests a functional interaction with the Golgi apparatus. The EBs then differentiate into RBs that undergo binary fission. After approx 36 hours, the RBs differentiate into EBs. At about 48 hours, release may occur by cytolysis or by a process of exocytosis or extrusion of the whole inclusion, leaving the host cell intact. Chlamydiae may also enter a persistent state after treatment with certain cytokines, such as y-interferon, treatment with antibiotics, or restriction of certain nutrients, although in the persistent state metabolic activity is reduced. The ability to cause prolonged, often subclinical, infection is one of the major characteristics of Chlamydiae.
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