Mycoplasmas: An overview
The History of Discovery
The first mycoplasma representatives – the causative agents of pleuropneumonia – were isolated by E. Nocard and E. Roux from the lungs of cattle.
In 1944 M. Eaton isolated a filterable agent from patient’s sputum, which caused pneumonia in animals (cotton rats). Further investigations carried out by R. Chanok, L. Haifflik, and M. Borrel confirmed that the Eaton’s agent belongs to the mycoplasmas.
- Mycoplasmas pertain to the separate phylum Tenericutes, class Mollicutes, order Mycoplasmatales, family Mycoplasmataceae, which includes two genera with pathogenic representatives: Mycoplasma and Ureaplasma. More than 200 mycoplasmal species are known to date.
- In humans several species of mycoplasma have evident clinical relevance. The most virulent agent Mycoplasma pneumoniae causes pneumonia; also this bacterium is associated with joint and some other infections.
- Ureaplasma urealyticum, Ureaplasma parvum, Mycoplasma hominis, and in certain cases Mycoplasma genitalium can cause human nongonococcal urethritis, especially in association with other bacteria.
Structure and Properties of Mycoplasmas
- Mycoplasmas are the smallest bacteria known being 125-250 nm in size. They pass through the filters with 0.45 μm pore size; hence, they are comparable to large viruses.
- Mycoplasmas are highly pleomorphic gram-negative bacteria (appear in rings, bacillary and spiral bodies, filaments, granules, etc.) because they have no cell wall.
- The cells of mycoplasmas are surrounded by thick triple-layered membrane containing large amounts of sterols (as the result, mycoplasmas require sterols for growth).
- They are lack of capsule; ureaplasmas can carry flagella.
- The optimum growth temperature for mycoplasmas is 36-37°C. Despite the fact that mycoplasmas are membrane parasites, they can grow on cell-free media that contain lipoproteins and sterols.
- For instance, many strains of mycoplasmas can be cultured in heart infusion peptone broth with 2% agar (pH 7.8) supplemented with human ascitic fluid or animal serum (horse, rabbit), as well as on blood, serum or ascitic agar. Mycoplasmas are resistant to thallium acetate that is used for inhibition of concomitant bacteria.
- After cultivation for 5-10 days or even more the round colonies of minimal sizes appear that have a granular surface and a dark center resembling “fried eggs”. Some strains can produce hemolysis.
- In cell cultures as well as in vivo mycoplasmas grow predominantly at the cell surfaces being attached to cytoplasmic membranes of the cells. Mycoplasmas readily multiply in the chorioallantoic membrane of chicken embryos.
- Mycoplasmas are facultative anaerobes or microaerophils. Overall, they have limited number of enzymes with reduced metabolism.
- Many mycoplasmas ferment glucose as the source of energy with acid end products; some strains utilize arginine, ureaplasmas require urea.
- Certain species are able to produce peroxides.
- Mycoplasmas acquire sterols for their growth directly from cellular membranes (membrane parasitism).
Basic antigens of mycoplasmas are glycolipids and proteins with variable structure. Microbial enzymes also demonstrate antigenic properties.
- The whole number of mycoplasmal virulence factors is not well elucidated yet. Mycoplasmas carry various adherence structures: interactive proteins, adhesins, and adherence-accessory proteins, responsible for microbial attachment.
- Adhesin P1 is the major virulence factor of M. pneumoniae that impairs the function of ciliated epithelium.
- Some strains were shown to produce toxin-like substances and hemolysins.
- Bacteria can generate cytotoxic hydrogen peroxide and superoxide radicals.
- Membrane compounds of mycoplasma play the role of superantigens.
Mycoplasmas are very sensitive to the environmental influences. They are easily inactivated by heating, drying, sunlight, UV irradiation, and pH fluctuations. The bacteria are destroyed under the action of conventional disinfectants.
Pathogenesis and Clinical Findings in Mycoplasmal Pneumonia
- The mycoplasmas appear to be rather host-specific, being contagious and virulent only for the specific host.
- Thus, the source of micoplasmal pneumonia is the sick person and mycoplasma carrier.
- The disease is transmitted by airborne route.
- Minimum infectious dose of bacteria to cause pneumonia is very low – about 100 microbial cells.
- M. pneumoniae attachs to the membranes of ciliated and nonciliated epithelium of respiratory tract (membrane parasitism). During infection, the bacteria remain extracellularly.
- Cytotoxic substances, free radicals and peroxides, microbial superantigens as well as immune complex-mediated cytolysis and cellular inflammation intensify the injury of respiratory epithelium mainly resulting in interstitial pneumonia.
- The incubation period of the disease varies from 1 to 3 weeks. Usually mycoplasmal pneumonia has moderate manifestations with torpid course of infection.
- The clinical spectrum of mycoplasmal pneumonia varies from asymptomatic infection to serious pneumonitis.
- The onset of the disease is usually faint with fatigue, dry cough, subfebrile temperature or fever, and sore throat.
- Initially the patient demonstrates only moderate illness. The physical signs of pulmonary inflammation are poorly determined but X-ray examination shows intensive lung involvement.
- Resolution of pneumonitis and clinical improvement are observed in 2-4 weeks.
- The presence of antibodies to M. pneumoniae is associated with resistance to infection. Cell-mediated immune reactions occur as well. The pneumonic process may be attributed in part to an immunologic response rather than only to infection by mycoplasmas.
- After manifested form of the disease the specific humoral and cellular immunity lasts for 5-10 years. Mild and subclinical cases of micoplasmal infections confer only short-term and low-grade immune response.
Laboratory Diagnosis of Mycoplasmal Pneumonia
- The diagnosis of pneumonia caused by M. pneumoniae can be largely made on the ground of clinical findings and X-rays.
- Laboratory testing has to confirm the clinical diagnosis.
- The basic clinical specimen is sputum.
- Rapid detection of bacteria is performed with immunofluorescence; microbial antigens are determined by ELISA.
- The most sensitive test for the detection of microbial DNA is PCR. It remains the method of choice for laboratory diagnosis of mycoplasmal infections.
- Culture isolation is the long-term and cumbersome technique; thereby it is performed mainly in reference laboratories.
- Serological diagnosis for mycoplasmal pneumonia is confirmed by ELISA in paired sera test. The fourfold increase of the titer of specific antibodies indicates ongoing infection.
Treatment and Prophylaxis of Mycoplasmal Pneumonia
- As micoplasmas are totally lack of the cell wall, they are endowed with the intrinsic resistance to all β-lactams. They are also resistant to sulfonamides because of inabilty to produce folic acid.
- Azalides, macrolides, and tetracyclines are the drugs of choice for treatment of mycoplasma-associated infections.
- None of specific vaccines are available now for clinical use.