Mollicutes is a class of bacteria distinguished by the absence of a cell wall. 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. Colonies show the typical "fried-egg" appearance. Mollicutes are 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. 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. Origin and development to parasitic life Analysis of the genomes of Mollicutes ("mycoplasmas") gives solid support for the hypothesis that Mollicutes have developed from Gram-positive bacteria by a process of reductive evolution. By adopting a parasitic mode of life with use of nutrients from their hosts, many Mollicutes were able to reduce their genetic material considerably. On the other hand, they lost the genes for many assimilative processes. Thus, Mycoplasma possibly became the smallest self-replicating organism in nature. Mycoplasma genitalium, with 580,000 base pairs, has an especially small genome size. Some phytoplasmas also have a very small genome size. The genera with the smallest genome are considered to be phylogenetically the most "recent" mollicutes. To maintain their parasitic mode of life the Mollicutes have developed rather sophisticated mechanisms to colonize their hosts and resist the host immune system. Proliferation Mollicutes (formerly also called pleuropneumonia-like organisms, PPLO) as well as L-form bacteria (formerly also called L-phase bacteria), both lacking cell walls, do not proliferate by binary fission but by a budding mechanism. In 1954, this mode of proliferation has been shown by continual observations of live cells using phase-contrast microscopy. Previously, Mollicutes were sometimes considered stable L-form bacteria, but phylogenetic analysis has identified them as bacteria that have lost their cell walls in the course of evolution. History of the classification The classification of the