Mycoplasma is a genus of bacteria that, like the other members of the class Mollicutes, lack a cell wall around their cell membranes. Peptidoglycan (murein) is absent. This characteristic makes them naturally resistant to antibiotics that target cell wall synthesis (like the beta-lactam antibiotics). They can be parasitic or saprotrophic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of "walking" pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. Mycoplasma species (like the other species of the class Mollicutes) are among the smallest organisms yet discovered, can survive without oxygen, and come in various shapes. For example, M. genitalium is flask-shaped (about 300 x 600 nm), while M. pneumoniae is more elongated (about 100 x 1000 nm), many Mycoplasma species are coccoid. Hundreds of Mycoplasma species infect animals.
The trivial name “mycoplasma” (plural mycoplasmas or mycoplasms) is commonly used for all members of the class Mollicutes. In scientific classification, the designation Mycoplasma refers exclusively to the genus, a member of the Mycoplasmataceae, the only family of the order Mycoplasmatales (see “scientific classification”).
Etymology
The term "mycoplasma", from the Greek μύκης, (fungus) and πλάσμα, (formed), was first used by Albert Bernhard Frank in 1889 to describe an altered state of plant cell cytoplasm resulting from infiltration by fungus-like microorganisms. Julian Nowak later proposed the name mycoplasma for certain filamentous microorganisms imagined to have both cellular and acellular stages in their lifecycles, which could explain how they were visible with a microscope, but passed through filters impermeable to other bacteria. Later, the name for these mycoplasmas was pleuropneumonia-like organisms (PPLO), broadly referring to organisms similar in colonial morphology and filterability to the causative agent (a Mycoplasma species) of contagious bovine pleuropneumonia. At present, all these organisms are classified as Mollicutes and the term Mycoplasma refers solely to the genus.
Species that infect humans
Species of Mycoplasma, other than those listed below, have been recovered from humans, but are assumed to have been contracted from a non-human host. The following species use humans as the primary host:
M. amphoriforme
M. buccale
M. faucium
M. fermentans
M. genitalium
M. hominis
M. incognitus
M. lipophilum
M. orale
M. penetrans
M. pirum
M. pneumoniae
M. primatum
M. salivarium
M. spermatophilum
Pathophysiology
Mycoplasma species have been isolated from women with bacterial vaginosis. M. genitalium is found in women with pelvic inflammatory disease. In addition, infection is associated with increased risk of cervicitis, infertility, preterm birth and spontaneous abortion. Mycoplasma genitalium has developed resistance to some antibiotics. Mycoplasma species are associated with infant respiratory distress syndrome, bronchopulmonary dysplasia, and intraventricular hemorrhage in preterm infants.
Characteristics
Over 100 species have been included in the genus Mycoplasma, a member of the class Mollicutes. They are parasites or commensals of humans, animals, and plants. The genus Mycoplasma uses vertebrate and arthropod hosts. Dietary nitrogen availability has been shown to alter codon bias and genome evolution in Mycoplasma and Phytoplasma.
Mycoplasma species are among the smallest free-living organisms (about 0.2 - 0.3 µm in diameter). They have been found in the pleural cavities of cattle suffering from pleuropneumonia. These organisms are often called MLO (mycoplasma-like organisms) or, formerly, PPLO (pleuropneumonia-like organisms).
Important characteristics of Mycoplasma species
Cell wall is absent and plasma membrane forms the outer boundary of the cell. Due to the absence of cell walls these organisms can change their shape and are pleomorphic. Lack of nucleus and other membrane-bound organelles. Genetic material is a single DNA duplex and is naked. Ribosomes are 70S type. Possess a replicating disc at one end which assists replication process and also the separation of the genetic materials. Heterotrophic nutrition. Some live as saprophytes but the majority are parasites of plants and animals. The parasitic nature is due to the inability of mycoplasmal bacteria to synthesise the required growth factor.