Chlamydiaceae

Chlamydiaceae form a family of pathogenic, obligate intracellular, Gram-negative bacteria that cause acute and chronic diseases in animals and humans (Pannekoek Y et al. 2006). They are distinguished from all other bacteria by their small size (smallest unit 0.2 μm), a special reproductive cycle that occurs only within the cell (obligate cell parasitism; chlamydiae are dependent on the cell's ATP for energy), and the existence of 2 distinct cell morphological appearances, elementary and intial bodies. Remark: As Chlamydia spp. are obligate intracellular bacteria, they can only be grown in special culture media (yolk sac of the chicken embryo) or cell cultures.

All chlamydiae possess a cell wall antigen(lipopolysaccharide - LPS), an antigen which has, among other things, diagnostic significance. Chlamydial LPS has been found to contain a chemically and antigenically unique structure of a trisaccharide of sequence α-Kdo-(2→8)-α-Kdo-(2→4)-α-Kdo-(2→. Monoclonal antibodies directed against this trisaccharide epitope are genus-specific and recognize only bacteria of the genus Chlamydia.

Furthermore, the"major outer-membrane protein" - MOMP - plays a major role in the integrity (structural rigidity of the outer membrane) and antigenicity of Chlamydia. MOMP trimers show porin-like channel-forming activity . The pore size was estimated to be about 2 nm in diameter. Together with lipopolysaccharide (LPS), MOMP forms the surface of the elementary body cell.

Within the family Chlamydiaceae, the genus Chlamydia is of particular importance for human medicine. A distinction is made between:

Chlamydia psittaci:

• Chlamydia psittaci finds its natural reservoir among animals, but infections in humans are known from avian strains, which cause severe pneumonia (psittacosis - better ornithosis) . C.psittaci was developed in the early 20th century by both the USA and the former Soviet Union as a means of biological warfare. It is spread by contact with birds (even accidentally) or other domestic animals (Stalder S et al 2020).

Chlamydia trachomatis:

• C. trachomatis is divided into 15 serovars - based on OmpA [MOMP] antigenicity).
• Serovars A through C, is the causative agent of trachoma.
• Serovars D through K cause inclusion body conjunctivitis (harmless variant of trachoma) also swimming pool conjunctivitis .
• Furthermore, serovars D to K cause sexually transmitted diseases in men (urethritis and prostatitis) and women (urethritis, cervicitis and salpingitis). Up to 60% of non-gonococcal urethritis (NGU) in men is caused by Chlamydia trachomatis. Reiter's syndrome can be caused as a postinfectious complication of this urinary tract infection.
• Serovar L1-3 causes lymphogranuloma venereum (LGV) and proctocolitis (LGV biovar). Strains of LGV biovar are able to penetrate beyond the mucosal epithelium and enter the lymphatics, where they proliferate in monocytes and macrophages.

Chlamydia pneumoniae (TWAR Chlamydia):

• C. pneumoniae emerges as a causative agent of atypical pneumonias and has recently been linked to the pathogenesis and clinical consequences of atherosclerosis. C. pneumoniae is transmitted from person to person as a specificity.

All Chlamdiaceae share a biphasic mode of life with 2 distinct developmental forms:

• EB: Elementary body (non-replicative elementary body) EBs are metabolically slowed, spore-like forms specialized for survival in the environment and attachment to host cells. EBs usually attach to non-phagocytic cells through a number of different receptors (CFTR, mannose receptor, etc.).
• RB: Initial body (replicative, non-infectious reticulate body -RB): After uptake (clathrin-mediated endocytosis) by an epithelial cell, EBs differentiate into RBs in the vacuoles of the host cell. RBs are the germinal forms that actively divide 8-10x (inclusion bodies) before differentiating back into EBs. EBs can exit the cell by cell lysis and reinfect new cells (Cossé MM et al. 2018).

Virulence determinants of Chlamydiaceae include numerous mechanisms for utilizing host cell nutrients and building blocks for cellular machinery.

Tetracyclines (e.g. doxycycline) and macrolide antibiotics (e.g. erythromycin) are suitable for the treatment of all chlamydia.

S. Guidelines of the German STI Society p.14

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