Mollicutes

Mollicutes is a class of bacteria distinguished by the absence of a cell wall and its peptidoglycan. The word "Mollicutes" is derived from the Latin mollis (meaning "soft" or "pliable"), and cutis (meaning "skin"). Individuals are very small, typically only 0.2–0.3 μm (200–300 nm) in size and have a very small genome size. They vary in form, although most have sterols that make the cell membrane somewhat more rigid. Many are able to move about through gliding, but members of the genus Spiroplasma are helical and move by twisting. The best-known genus in the Mollicutes is Mycoplasma, though it has been split in 2018 into several genera. Colonies show the typical "fried-egg" appearance.

Mollicutes can be parasitic or saprotrophic. They can be parasites of various animals and plants, living on or in the host's cells. Many cause diseases in humans, attaching to cells in the respiratory or urogenital tracts, particularly species of Mycoplasma and Ureaplasma. Phytoplasma and Spiroplasma are plant pathogens associated with insect vectors.

The absence of peptidoglycan makes them naturally resistant to antibiotics such as the beta-lactam antibiotics that target cell wall synthesis. Several species are pathogenic in humans, including Mycoplasmoides pneumoniae, which is an important cause of "walking" pneumonia and other respiratory disorders, and Mycoplasmoides genitalium, which is believed to be involved in pelvic inflammatory diseases. Mollicutes species are among the smallest organisms yet discovered, can survive without oxygen, and come in various shapes. For example, Md. genitalium is flask-shaped (about 300 x 600 nm), while Md. pneumoniae is more elongated (about 100 x 1000 nm), many Mollicutes species are coccoid. Hundreds of Mollicutes species, mainly those formerly classified in Mycoplasma, infect animals.

Whereas formerly the trivial name "mycoplasma" (plural: "mycoplasmas") has commonly denoted any member of the class Mollicutes, it now in scientific classification refers exclusively to a member of the genus Mycoplasma. A reorganization of Mycoplasma in 2018 has moved many species out of the genus, making this old interpretation once again useful. The smallest taxonomic node that covers all members of pre-2018 Mycoplasma is Mollicutes.