GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

7. PROKARYOTE SYSTEMATICS

7.5. CHARACTERISTICS OF TAXA ACCORDING TO THE NINTH EDITION OF BERGEY'S MANUAL OF SYSTEMATIC BACTERIOLOGY

7.5.3. Phylum Tenericutes

This phylum comprises prokaryotes lacking a Cell wall (referred to as mycoplasms) and incapable of synthesizing peptidoglycan precursors. Their Cells are bounded by a elementary Plasma Membrane, are pleomorphic, and vary in size (ranging from a minimum of 0.15-0.20 to a maximum of 10 µm). Characteristic branched filamentous forms occur. Reproduction may proceed via budding, fragmentation, and/or binary fission. They are non-motile, although gliding motility is characteristic of certain species. They are Gram-negative. Complex nutrient media (media with high osmotic pressure) are required for growth. These organisms sometimes resemble bacterial L-forms that lack a cell wall (which frequently arise in Gram-positive eubacteria); however, they differ from these eubacteria in being unable to revert to the normal type and synthesize a cell wall. Many species additionally require Cholesterol and long-chain Fatty acids for growth. The DNA G+C content is 23-46 %.

The absence of a cell wall determines the structural peculiarities of Bacteria in this group: 1) cellular polymorphism — spherical, ellipsoidal, disc-shaped, rod-shaped, filamentous, and branched forms; 2) resistance to Antibiotics that specifically target The Cell wall (e.g., penicillin and its analogs); 3) the presence of a well-developed, stable, and elastic plasma membrane.

They are grouped into the Class Mollientes (consisting of a single subdivision — No. 21). In Bergey's Manual of Determinative Bacteriology (1997), mycoplasms are placed in group No. 30. Representatives of this class are the smallest prokaryotes capable of independent reproduction. They are grouped into the order Mycoplasmetales and three families: Mycoplasmataceae, Acholeplasmataceae, and Spiroplasmataceae. The Classification of mycoplasms into families is based on their sterol requirements: sterol-dependent mycoplasms of the family Mycoplasmataceae require exogenous cholesterol (and other sterols), which is a major component of the Membrane Lipids in parasitic mycoplasms; mycoplasms of the family Acholeplasmataceae are sterol-independent.

Energy is derived from the oxidation or Fermentation of Organic compounds (mono- and Polysaccharides) and Inorganic Compounds (iron, manganese). The absence of Quinones and Cytochromes indicates that their Respiratory Chain is limited. Most are aerobes, alongside obligate anaerobes. Acidophiles capable of growing exclusively under highly acidic conditions also occur, such as Thermoplasma acidophilum, at pH 1-4. Representatives of the genus Thermoplasma are thermophiles, and certain mycoplasms are capable of hydrolyzing urea (genus Ureaplasma).

Among mycoplasms, saprophytic, parasitic, and pathogenic forms have been identified. The latter cause diseases in animals, plants, and tissue cultures.

In animals, mycoplasms occur as relatively harmless parasites on the mucous membranes of the respiratory and reproductive tracts. These organisms function as membrane parasites, meaning they attach quite firmly to the epithelial cells of mucous membranes. They do not secrete toxins; however, due to the absence of cell walls, even mildly toxic products such as ammonium ions and hydrogen peroxide exert adverse effects on the membranes of damaged animal cells. Mycoplasma infection may remain completely asymptomatic in some animals, whereas others develop inflammation of the respiratory tract and Lungs.

In plants, mycoplasms are the causative agents of yellows diseases. They localize primarily in the phloem and, due to their superficial resemblance to spirilla, are grouped into the genus Spiroplasma. Spiroplasma citri is the CAUSATIVE AGENT OF citrus yellows. Similar forms have been discovered in other plants (corn, bermudagrass, rice). Spiroplasma has also been isolated from bees and grasshoppers. It is likely that insects act not only as vectors for mycoplasms but also as their hosts.