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 which 15 are isolated from humans. Key human pathogens belong to the genera Mycoplasma and Ureaplasma.
Their key structural feature is the complete absence of a cell wall. The cell is covered exclusively by a cytoplasmic membrane, which incorporates sterols for added stability. A microcapsule is occasionally present on the outer surface. Due to the lack of a rigid scaffolding, these bacteria exhibit high osmotic sensitivity and extreme pleomorphism. In smears, they may appear as filamentous, flask-shaped, or coccoid forms.
Morphologically, mycoplasmas resemble bacterial L-forms when observed under phase-contrast microscopy. They do not form spores, are Gram-negative (staining poorly), and easily pass through bacterial filters.
Cultural Properties and Biochemical Activity
Regarding respiration, mycoplasmas are facultative anaerobes with an optimal temperature of 37 °C. They are extremely fastidious microorganisms: they require complex media supplemented with heart extracts, brain extracts, serum, and yeast extracts for growth.
A mandatory component of the growth medium is cholesterol, which mycoplasmas incorporate into their own cell membrane. Culturing Ureaplasma additionally requires urea. Alternative cultivation methods include embryonated chicken eggs and cell cultures.
Mycoplasmas grow slowly, taking 49 to 96 hours.
- On solid media, they form small "fried-egg" colonies: their opaque center grows deep into the agar, while the periphery remains flat and translucent.
- In liquid media, growth often occurs without visible turbidity, so it is detected either by subculturing or by sediment microscopy.
Biochemical activity within the group is low. Most species ferment glucose to produce acid. A differentiating feature of the genus Ureaplasma is the presence of urease, as they utilize urea. Mycoplasma growth is inhibited by specific immune sera.
Pathogenicity Factors and Mechanism of Infection
Pathogenic mycoplasmas are most often flask-shaped or filamentous. A specialized terminal structure is located at the cell pole, which is critical for virulence. It consists of a complex of adhesin proteins and accessory proteins. Accessory proteins are rich in proline, ensuring the proper folding of adhesins.
Mechanism of host cell damage:
- Attachment (adhesion) to species-specific glycolipid receptors on the human cell membrane.
- The bacterium begins to produce hydrogen peroxide ($H_2O_2$), peroxide radicals, and hemolysins.
- Damage occurs to the host cell membrane, followed by closer fusion of the microbial and host cell membranes.
- As a result, the metabolism of the infected cell is disrupted, and the protective function of the ciliated epithelium is impaired.
Ecology, Resistance, and Clinical Significance
The antigenic structure of mycoplasmas is represented by species-specific glycolipid and protein antigens. In the environment, the bacteria demonstrate relative stability, frequently inhabiting soil and stagnant water. However, they are sensitive to standard concentrations of disinfectants and antiseptics.
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 found in the urogenital tract (detectable in 30–70% of healthy women and 40–80% of sexually active individuals).
Key diseases caused by mycoplasmas (anthroponotic infections):
- Respiratory mycoplasmosis: caused by M. pneumoniae (atypical pneumonia).
- Urogenital infections: caused by M. hominis, M. genitalium, U. urealyticum.
- Neonatal pathology: premature infants are particularly susceptible (frequently infected with M. hominis and U. urealyticum).
Infection confers only short-lived immunity.