Edward and Freundt 1967
The Mollicutes are 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"). They are parasites of various animals and plants, living on or in 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, 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, nowadays it refers exclusively to a member of the genus Mycoplasma.
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. 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.
History of the classification
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 it was not clear whether they were fungi, viruses or bacteria. 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 the species Mycoplasma were still unidentified.
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 recognized as not 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.
Three divisions of the kingdom "Procaryotae"
Traditionally the taxonomy of bacteria was based on similarities and differences in morphology (Linnaean taxonomy).
In 1962 R.G.E. Murray proposed to divide the Kingdom Bacteria into three divisions (= phyla) on the basis of the cell wall types:
- Gram-negative "Gracilicutes", with a thin cell wall and little peptidoglycan;
- Gram-positive "Firmacutes", with a thicker cell wall and more peptidoglycan (the name was later changed in "Firmicutes"), and
- the "Mollicutes", without a cell wall.
The present taxonomy of mollicutes is based on: 1. molecular analysis, especially of 16S rRNA; 2. serology (reactions on antiserum) and 3. phenotype (morphology, requirements for growth).
For classification and nomenclature of species there are special rules, which are maintained by the International Committee on Systematics of Prokaryotes (ICSP) Subcommittee on the Taxonomy of Mollicutes (formerly the International Committee on Systematic Bacteriology (ICSB) Subcommittee on taxonomy of Mycoplasmatales).
Traditional versus phylogenetic classification
The classification of organisms can be made on the basis of morphology (Linnaean taxonomy) or according to ancestry/genetic origin (cladistics). In taxonomy, bacteria are preferably classified according to their descent.
Although the group Mollicutes phenotypically (without a cell wall and therefore resistant to antibiotics like penicillin and not Gram-positive) clearly is distinguished from Gram-positive bacteria with a cell wall, the morphology does not say anything about their position in the phylogenetic tree. Some evolutionary solutions, like wall-lessness, can arise (and become extinct) several times, even within a lineage. For example, the unrelated Thermoplasmas also lack a cell wall. For that reason, lack of a cell wall cannot be a valid criterion to define Mollicutes as a phylogenetically coherent group, or to determine whether a species developed earlier or later in the evolution.
The phylum for Mollicutes
The phylum in which the class Mollicutes should be placed is controversial. Some taxonomists place them in Firmicutes, others in Tenericutes. In effect, however, ascribing a phylum to the class Mollicutes is phylogenetically a minor issue, as it does not affect the phylogenetic trees within the Mollicutes.
The results of molecular phylogenetics are not straightforward. Beside the limitation of the amount of available data there are also intrinsic limitations of the used methods, that give uncertainties about the evolutionary development. The results partly depend on the chosen molecular marker, like rRNA, elongation factor or another protein.
Moreover, cladistic analyses are often based on assumptions that are hidden in the used computational algorithms, with techniques like maximum parsimony and likelihood. Such elements do have a somewhat arbitrary character.
This is illustrated by the fact that in an early edition of Bergey’s Manual of Systematic Bacteriology the Mollicutes are a class within the phylum Firmicutes, whereas in the announced 2nd edition, vol. 3 they are moved to a separate phylum Tenericutes.
The change is motivated by "their unique phenotypic properties, in particular the lack of rigid cell walls, and the general low support by alternative markers".
Woese et al. suggested that the Mollicutes might have been derived from different branches of bacteria. They concluded, that the Mollicutes are not a phylogenetically coherent group and therefore do not form a distinct higher level taxon. Instead, they cluster within Gram-positive bacteria of the phylum Firmicutes.
Phylogenetic trees based on phosphoglycerate kinase (Pgk) amino acid sequences indicated a monophyletic origin for the Mollicutes within the Firmicutes.
In the Taxonomic Outline of Bacteria and Archaea (TOBA Release 7.7), March 2007, the Mollicutes are a class in the phylum Firmicutes.
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