Mycoplasma

Taxonomy and morphological features

Mycoplasmas belong to the class Mollicutes (soft-skinned) of the phylum Tenericutes. This is a vast group including 4 families, 6 genera, and about 150 species of microorganisms, of which 15 species are isolated from humans. Key human pathogens belong to the genera Mycoplasma and Ureaplasma.

Their main anatomical feature is the complete absence of a cell wall. The cell is covered exclusively by a cytoplasmic membrane, which contains sterols to compensate for strength. Sometimes a microcapsule is found externally. Due to the lack of a rigid framework, these bacteria are characterized by high osmotic sensitivity and extreme polymorphism (pleomorphism). In smears, they may appear as filamentous, flask-shaped, or coccoid forms.

Externally, mycoplasmas morphologically resemble L-forms of bacteria when examined under a phase-contrast microscope. They do not form spores, are Gram-negative (stain negatively), and are easily filtered through bacterial filters.

Cultural properties and biochemical activity

By type of respiration, mycoplasmas are facultative anaerobes with an optimal temperature of 37 °C. These are extremely "fastidious" microorganisms: for growth, they require complex media based on heart, brain, serum, and yeast extracts.

An obligatory component of the nutrient medium is cholesterol, which mycoplasmas need to build their own membrane. Urea is additionally required for the cultivation of ureaplasmas. Alternative methods of cultivation include chicken embryos and cell cultures.

Mycoplasmas grow slowly — from 49 to 96 hours.

• On solid media, they form small "fried egg"-like colonies: their opaque center grows deeply into the agar, while the periphery remains round and translucent.
• In liquid media, growth often occurs without visible turbidity, so it is detected either by subculturing or by sediment microscopy.

Biochemical activity in the group is low. Most utilize glucose with acid production. A differential feature of the genus Ureaplasma is the presence of urease — they utilize urea. Mycoplasma growth is inhibited by specific immune sera.

Pathogenicity factors and mechanism of infection

Pathogenic forms of mycoplasmas are most often flask-shaped or filamentous. A specific terminal structure, critical for virulence, is located at the cell pole. It consists of a complex of adhesin proteins and auxiliary (accessory) proteins. Accessory proteins are rich in proline, which ensures proper folding of adhesins.

Mechanism of host cell damage:

1. Attachment (adhesion) to species-specific glycolipid receptors on the human cell membrane.
2. The bacterium begins to produce hydrogen peroxide ($H_2O_2$), peroxide radicals, and hemolysins.
3. Damage to the host membrane occurs, followed by closer fusion of the microbial and host cell membranes.
4. As a result, the metabolism of the infected cell is disrupted, and the defensive function of the ciliated epithelium is suppressed.

Ecology, resistance, and clinical significance

The antigenic structure of mycoplasmas is represented by species-specific glycolipid and protein antigens. In the environment, bacteria demonstrate relatively high resistance, often inhabiting soil and standing water. However, they are sensitive to disinfectants and antiseptics at standard concentrations.

Many species are part of the normal human microflora. For example, M. salivarium and M. orale colonize the oral cavity, while M. hominis and U. urealyticum are detected in the urogenital system (including in 30–70% of healthy women and 40–80% of sexually active people).

Main diseases caused by mycoplasmas (anthroponotic infections):

• Respiratory mycoplasmosis: pathogen M. pneumoniae (atypical pneumonia).
• Urogenital infections: pathogens M. hominis, M. genitalium, U. urealyticum.
• Neonatal pathology: premature infants are especially susceptible (more often infected with M. hominis and U. urealyticum).

Only short-lived immunity is formed after the infection.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cytotoxic and cytolytic effect of mycoplasma peroxide radicals on the host cell
• Mechanisms of long-term persistence of "membrane parasites" in the body
• The role of proline-rich accessory proteins in adhesin folding
• Alternative cultivation methods (chicken embryos and cell cultures)

Other topics in the section "Morphology and Classification of Microbes"

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