Morphology and Ultrastructure of Chlamydiae
Chlamydiae as well as rickettsiae pertain to obligate intracellular parasites.
The order Chlamydiales includes the family Chlamydiaceae; pathogenic represenatives are present within the genera Chlamydia and Chlamydophila.
These bacteria are of rather small sizes and they have two stages in their life cycle – elementary bodies and reticulate bodies.
According to the compositon of the cell wall, chlamydiae are gram-negative bacteria. Their cell walls contain LPS and elevated amounts of lipids. However, they have no typical peptidoglycan because of lack of acetylmuramic acid. On the other hand, they carry multiple cysteine-enriched peptide cross-bridges that make bacterial envelope rigid.
Elementary bodies are the round-shaped structures of minimal sizes (0.2-0.4 μm). They possess infectious properties being capable of invading eukaryotic cells. Before the entry into the cell, chlamydiae are metabolicaly inert.
Within the cells, elementary bodies transform into feeding vegetative forms, known as reticulate bodies. They actively reproduce nearby the nucleus of the cells, making characteristic inclusions covered by common membrane (Gk. chlamyda – mantle or cloak). The sizes of reticulate bodies are about 0.8-1.5 μm. After several reproductions they convert again into elementary invasive forms that leave the cell across cytoplasmic membrane. The total chlamydial life cycle takes near 3 days. The reproduction of chlamydiae is asynchronous, as all of microbial forms (elementary, reticulate, and intermediate) can be found in the cell at the same time.
Chlamydiae grow in cultures of eukaryotic cell lines. McCoy cells are commonly used for their culture. Likewise, they propagate within the yolk sac of embryonated chicken eggs and in laboratory animals (e.g., mice).
Chlamydiae are detected by Romanowsky-Giemsa stain (reticulate bodies produce blue inclusions, attached to cell nuclear membrane, whereas elementary bodies stain purple).
The monolayers of cell cultures are examined for chlamydial infection by direct immunofluorescence.
Chlamydiae cause numerous diseases in humans.
Chlamydia trachomatis is the causative agent of trachoma that afflicts eyes. It also cause sexually transmitted urogenital infections (in men – urethritis, epididymitis; in women – cervicitis, cystitis, salpingitis, pelvic inflammatory disease and others); venereal disease lymphogranuloma venereum; inclusion conjunctivitis of the newborns; arthritis and other diseases.
Chlamydophila pneumoniae causes chlamydial pneumonia; Chlamydophila psittaci is the agent of zoonotic avian infection ornithosis (or psittacosis) that may provoke severe lung or generalized infection in humans.