Two forms of existence
Chlamydia exhibit marked polymorphism. During their development, the microorganism exists in two radically different forms.
| Sign | Elementary bodies (EB) | Reticulate bodies (RB) |
|---|---|---|
| Function | Infectious form (transmission) | Vegetative form (growth and division) |
| Localization | Extracellular | Intracellular (usually near the nucleus) |
| Morphology | Small (0.2–0.3 µm), spherical | Large (up to 1.2 µm), ovoid |
| Staining (Romanowsky-Giemsa) | Red color | Blue or purple color |
| Structural features | Thick cell wall (inner and outer membranes), lacks N-acetylmuramic acid | Metabolically active, capable of binary fission |
Elementary bodies are highly resistant to the external environment: they withstand freezing down to –70 °C and prolonged drying, but are rapidly inactivated by heat and disinfectants.
Life cycle of Chlamydia
The reproduction process occurs intracellularly, predominantly in epithelial tissues, and includes strictly sequential stages:
- Adsorption and penetration. The elementary body attaches to the target cell and enters via endocytosis, ending up in a phagosome.
- Protection from lysis. This is a critical survival moment: chlamydial proteins block the fusion of the phagosome with cellular lysosomes, avoiding contact with destructive enzymes.
- Transformation and division. Inside the vacuole, the EB transforms into a metabolically active reticulate body. RBs begin repeated binary fission, forming intracellular inclusions.
- Maturation and release. Reticulate bodies condense, undergoing reverse differentiation into elementary bodies. Finally, the membranes of the vacuole and the host cell rupture (lysis), and new infectious forms emerge.
Classification and clinical significance
The family Chlamydiaceae includes human-pathogenic genera Chlamydia and Chlamydophila. Species differ in antigenic structure (serovars) and resulting pathologies:
- Chlamydia trachomatis (anthroponosis, 20 serovars). Serovars A–C cause trachoma (a severe eye infection leading to blindness due to corneal scarring). Serovars D–K are responsible for urogenital chlamydia, which can lead to adhesions, infertility, and ectopic pregnancy; in newborns, they cause pneumonia and conjunctivitis. Serovars L1–L3 are the causative agents of lymphogranuloma venereum.
- Chlamydophila psittaci (zooanthroponosis, 10 serovars). Causes psittacosis (ornithosis) — a severe lung infection. Interestingly, humans are a "dead end" for this infection (person-to-person transmission is absent).
- Chlamydophila pneumoniae (anthroponosis, 1 serovar). Provokes acute respiratory infections (bronchitis, pneumonia, pharyngitis). Chronic persistence is associated with the development of atherosclerosis, bronchial asthma, and sarcoidosis.
Metabolism and pathogenicity factors
Chlamydia occupy a unique position between viruses and classical bacteria. They have their own ribosomes and protein synthesis systems, and they can ferment pyruvic acid. However, they completely lack oxidative phosphorylation systems and do not synthesize high-energy compounds. Because of this, they are forced to use the host cell's ATP, earning them the name "energy parasites".
The main pathogenicity factors are:
- Adhesins — outer membrane proteins of elementary bodies.
- Endotoxin — genus-specific lipopolysaccharide (LPS) of the cell wall.
- Antiphagocytic proteins — prevent digestion inside macrophages.
- Heat shock proteins (HSP) — capable of triggering autoimmune reactions. A striking example is Reiter's syndrome, occurring in men against the background of urogenital chlamydia.
Briefly: answers to topical questions
What laboratory diagnostic methods are used to detect Chlamydia?
A comprehensive set of methods is used to detect Chlamydia, including microscopic, serological, and molecular biological studies.
- Culture method (cultura) — isolation of intracellular parasites in cell culture.
- Immunological methods (serologia) — immunofluorescence (IFA), enzyme-linked immunosorbent assay (ELISA), and complement fixation test (CFT).
- Molecular biological method (PCR) — polymerase chain reaction for the detection of pathogen DNA.
Which antibiotic groups are used to treat chlamydial infections?
The following groups of antibacterial drugs are used for etiotropic therapy of chlamydial infections:
- Macrolides (macrolidi) — azithromycin, roxithromycin, spiramycin, josamycin, erythromycin, clarithromycin, and midecamycin.
- Tetracyclines (tetracylini) — doxycycline.
- Fluoroquinolones (fluorquinoloni) — ofloxacin.
Beta-lactam antibiotics are not used due to the absence of a complete peptidoglycan layer in Chlamydia.
What are the routes and mechanisms of transmission of Chlamydia psittaci?
The causative agent of ornithosis, Chlamydophila psittaci, is transmitted to humans from infected birds. Ornithosis is classified as a zooanthroponosis. Person-to-person transmission is absent.
Elementary bodies — Exported (leave the cell, infectious), Reticulate — Multiply (intracellular, vegetative).
Class and exam questions
Why do chlamydiae fail to grow on simple nutrient media?
Due to energy parasitism and the inability to synthesize their own ATP, they can only replicate inside living cells. Laboratories use chicken embryos, laboratory animals, or special cell lines (HeLa, McCoy).
What is the purpose of preparing cell cultures before infection with Chlamydia?
Cells are pre-irradiated or treated with cycloheximide (a protein synthesis inhibitor). This suppresses the metabolism of the host cell, which facilitates Chlamydia's capture and assimilation of foreign ATP.
What are the main antigens of Chlamydia?
The genus-specific antigen is identified as the lipopolysaccharide (LPS) of the outer membrane, alongside species-specific antigens associated with the major outer membrane protein (MOMP).
What else to study on this topic
- Detailed structure of the chlamydial cell wall (absence of N-acetylmuramic acid in peptidoglycan)
- Pathogenesis of Reiter's syndrome and the role of heat shock proteins (HSP)
- Mechanisms of blocking phagosome-lysosome fusion
- Features of infection generalization and persistence in lymphoid tissue
- Differences between zooanthroponoses (psittacosis) and anthroponotic chlamydial infections
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