Mollicutes is a class of bacteria[2] 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.

Scientific classification
Domain: Bacteria
Phylum: Tenericutes
Class: Mollicutes
Edward and Freundt 1967[1]


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.

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.[3]

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, 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.[4]

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.[5] In 1967 the class Mollicutes, containing the order Mycoplasmatales, was proposed by the Subcommittee on Taxonomy of the Mycoplasmata.[1] 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:

  1. Gram-negative Gracilicutes, with a thin cell wall and little peptidoglycan;
  2. Gram-positive "Firmacutes", with a thicker cell wall and more peptidoglycan (the name was later changed in "Firmicutes"), and
  3. the "Mollicutes", without a cell wall.[6]

Modern taxonomy

Phylogenetic position of Mollicutes among bacteria, using 16S rRNA sequences.[7]

For classification and nomenclature of Mollicutes, 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).[8]

Traditionally, Mollicutes taxonomy has been based on serology and phenotypic characteristics. However, most modern classifications are based on DNA or RNA sequences, especially 16S rRNA sequences[9] (see Figure).

The phylum for Mollicutes

The results of Mollicutes phylogenetic analyses have been controversial. Some taxonomists place them in Firmicutes, others in Tenericutes. 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.[10] The results of molecular phylogenetic analyses have partly depend on the chosen molecular marker, like rRNA, elongation factor or another protein.[11] Phylogenetic trees based on phosphoglycerate kinase (Pgk) amino acid sequences' indicated a monophyletic origin for the Mollicutes within the Firmicutes.[12]

An early edition of Bergey's Manual of Systematic Bacteriology placed class Mollicutes within phylum Firmicutes,[13][14] whereas in the announced 2nd edition, they are moved to a separate phylum Tenericutes.[15][16][17] 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".[11] In the Taxonomic Outline of Bacteria and Archaea (TOBA Release 7.7), March 2007, the Mollicutes are a class in the phylum Firmicutes.[18]


  1. Edward, D.G.; F.A. Freundt (July 1967). "Proposal for Mollicutes as name of the class established for the order Mycoplasmatales" (PDF). Int J Syst Bacteriol. 17 (3): 267–268. doi:10.1099/00207713-17-3-267.
  2. "Mollicutes", from Encyclopædia Britannica Online. Retrieved 15 Oct. 2011.
  3. Shmuel Razin, David Yogev and Yehudith Naot Molecular Biology and Pathogenicity of Mycoplasmas. Micr. and Molec. Biology Reviews, December 1998, pp. 1094–1156, Vol. 62, No. 4
  4. Hayflick L. & Chanock, R.M. (1965). "Mycoplasma Species of Man" (PDF). Bacteriological Reviews. 29 (2): 185–221.
  5. Edward DG, Freundt EA (February 1956). "The classification and nomenclature of organisms of the pleuropneumonia group". J. Gen. Microbiol. 14 (1): 197–207. doi:10.1099/00221287-14-1-197. PMID 13306904.pdf
  6. N.E. Gibbons & R.G.E. Murray Proposals Concerning the Higher Taxa of Bacteria Int J Syst Bacteriol Vol. 28 (1) Jan. 1978, pp. 1–6.
  7. Oshima, K; Maejima, K; Namba, S (2013). "Genomic and evolutionary aspects of phytoplasmas". Frontiers in Microbiology. 4: 230. doi:10.3389/fmicb.2013.00230. PMC 3743221. PMID 23966988.
  8. Revised minimal standards for description of new species of the class Mollicutes (division Tenericutes) PDF ; Daniel R. Brown, Robert F. Whitcomb and Janet M. Bradbury (2007) Int J Syst Evol Microbiol 57 2703–2719; doi:10.1099/ijs.0.64722-0
  9. C.R. Woese, J. Maniloff and L.B. Zablen Phylogenetic analysis of the mycoplasmas Proc. Natl. Acad. Sci. USA Vol. 77, No. 1, pp. 494–498, January 1980
  10. C.R. Woese, J. Maniloff and L.B. Zablen Phylogenetic analysis of the mycoplasmas Proc. Natl. Acad. Sci. USA Vol. 77, No. 1, pp. 494–498, January 1980
  11. Ludwig, W. and Schleifer, K.H. Molecular phylogeny of bacteria based on comparative sequence analysis of conserved genes Microbial phylogeny and evolution, 2005, p. 70-98.
  12. Wolf, Matthias; Müller, T; Dandekar, T; Pollack, JD; et al. (2004). "Phylogeny of Firmicutes with special reference to Mycoplasma (Mollicutes) as inferred from phosphoglycerate kinase amino acid sequence data". Int J Syst Evol Microbiol. 54 (Pt 3): 871–875. doi:10.1099/ijs.0.02868-0. PMID 15143038. DOI 10.1099/ijs.0.02868-0
  13. Taxonomic Outline of the Prokaryotes. Bergey's Manual of Systematic Bacteriology. Archived 2009-12-29 at the Wayback Machine Garrity, Bell & Lilburn; Second Edition, Release 5.0 (2004); pp. 140–204. doi:10.1007/bergeysoutline200405.
  14. K. Johansson and B. Pettersson, Taxonomy of Mollicutes (2002)
  15. "Tenericutes". Taxonomy Browser. NCBI. Retrieved 2008-09-11.
  16. Wolfgang Ludwig, Karl-Heinz Schleifer and William B. Whitman (In press, release in 2009). "Revised road map to the phylum Firmicutes". In P. De Vos et al. (eds.) Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 3 (The Firmicutes). Springer-Verlag, New York. ISBN 0-387-95041-9; PDF Archived 2010-09-21 at the Wayback Machine
  17. Validation of the Publication of New Names and New Combinations ... Int. J. Syst. Bacteriol.; July 1984, pp. 355–357
  18. Garrity, et al. The Taxonomic Outline of Bacteria and Archaea (TOBA Release 7.7); March 2007. "Taxonomic Outline of Bacteria and Archaea" (PDF). Taxonomic Outline of Bacteria and Archaea. Archived from the original (PDF) on 2011-10-07. Retrieved 2009-08-13.

Mollicutes at the US National Library of Medicine Medical Subject Headings (MeSH)

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