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The Mollicutes are a class of distinguished by the absence of a cell wall. They are primarily parasites of various animals and plants, living within the host's cells. Individuals are very small, typically only 0.2-0.3 µm 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 Mollicutes is Mycoplasma.

Many Mollicutes cause diseases in humans, entering cells in the respiratory or urogenital tracts. These include certain species of Mycoplasma, Ureaplasma and Erysipelothrix.

Phytoplasma and Spiroplasma are plant pathogens associated with insect vectors.

Origin and Development To Parasitic Life

Analysis of the genomes of mycoplasmas gives solid support for the hypothesis that mycoplasmas 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 mycoplasmas were able to reduce their genetic material considerably. On the other hand mycoplasma lost the genes for many assimilative processes. Thus Mycoplasma possibly became the smallest self-replicating organism in nature. Especially Mycoplasma genitalium has with 580,000 base pairs a very small genome size. Some phytoplasmas also hava 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 develop ed rather sophisticated mechanisms to colonize their hosts and resist the host immune system. ^[4]

Taxonomy

In 1967 the Mollicutes were named as a class, with the order Mycoplasmatales.^[3] Afterwards they became a class of the Firmicutes. ^{[5] [6]} In the 2nd edition, vol. 3 of Bergey's Manual of Systematic Bacteriology the Mollicutes are currently classified within the phylum Tenericutes. ^[7]. ^[1]

Formerly the name Mycoplasma was used as a synonym of any member of Mollicutes. Nowadays Mycoplasma is one genus in the class Mollicutes.