Mollicutes

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. 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' has commonly denoted any member of the class Mollicutes, it now refers exclusively to a member of the genus Mycoplasma. 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. 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. The classification of the Mollicutes has always been difficult. The individuals are tiny, and being parasites, they have to be cultivated on special media. Until now, many species could not be isolated at all. In the beginning, whether they were fungi, viruses, or bacteria was not clear. Also, the resemblance to L-forms was confusing. At first, all members of the class Mollicutes were generally named 'mycoplasma' or pleuropneumonia-like organism (PPLO). Mollicutes other than some members of genus Mycoplasma were still unidentified. The first species of Mycoplasma/Mollicutes, that could be isolated was Mycoplasma mycoides. This bacterium was cultivated by Nocard and Roux in 1898. In 1956, D.G. Edward and E.A. Freundt made a first proposal for classifying and naming PPLOs. They left undecided, however, whether they belong to the bacteria (prokaryotes, in 1956 called 'Schizomycetes') or to the eukaryotes. As type species (name-giving species) of the PPLOs/mycoplasmas, Edward and Freundt proposed Mycoplasma mycoides, being the causative organism of bovine pleuropneumonia and referring to the pleuropneumonia-like organisms. Until then, Mycoplasma mycoides was known as Asterococcus mycoides, but later that name was not recognized as valid. In their publication of 1956, they described 15 species of Mycoplasma. In 1967 the class Mollicutes, containing the order Mycoplasmatales, was proposed by the Subcommittee on Taxonomy of the Mycoplasmata. Now, the name Mycoplasma should exclusively be used for members of the genus Mycoplasma, rather than the use as a trivial name for any mollicute. As the trivial name has been used in literature for a long time, this is yet not always the case. Traditionally, the taxonomy of bacteria was based on similarities and differences in morphology (Linnaean taxonomy).