Mollicutes
Encyclopedia
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 sterol
s 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.
, 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.
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.
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 (prokaryote
s, in 1956 called "Schizomycetes") or to the eukaryote
s. 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.
(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:
; 2. serology (reactions on antiserum) and 3. phenotype (morphology, requirements for grow).
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).
) 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 Thermoplasma
s 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 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".
The phylum Tenericutes was already validly published in 1984 by R.G.E. Murray.
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.
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
distinguished by the absence of a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
. 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 sterol
Sterol
Sterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol...
s that make the cell membrane somewhat more rigid. Many are able to move about through gliding
Bacterial gliding
Bacterial gliding is a process whereby a bacterium can move under its own power. This process does not involve the use of flagella, which is a more common means of motility in bacteria...
, but members of the genus Spiroplasma
Spiroplasma
Spiroplasma is a genus of Mollicutes, a group of small bacteria without cell walls. Spiroplasma shares the simple metabolism, parasitic lifestyle, fried-egg colony morphology and small genome of other Mollicutes, but has a distinctive helical morphology, unlike Mycoplasma. It has a spiral shape...
are helical and move by twisting. The best-known genus in Mollicutes is Mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
.
Many Mollicutes cause diseases in humans, attaching to cells in the respiratory or urogenital tracts, particularly species of Mycoplasma and Ureaplasma
Ureaplasma urealyticum
Ureaplasma urealyticum is a bacterium belonging to the family Mycoplasmataceae. Its type strain is T960.-Clinical significance:U. urealyticum is part of the normal genital flora of both men and women...
.
Phytoplasma
Phytoplasma
Phytoplasma are specialised bacteria that are obligate parasites of plant phloem tissue and transmitting insects . They were first discovered by scientists in 1967 and were named mycoplasma-like organisms or MLOs. They cannot be cultured in vitro in cell-free media...
and Spiroplasma are plant pathogens associated with insect vectors.
Whereas formerly the trivial name
Trivial name
In chemistry, a trivial name is a common name or vernacular name; it is a non-systematic name or non-scientific name. That is, the name is not recognised according to the rules of any formal system of nomenclature...
"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 genitaliumMycoplasma genitalium
Mycoplasma genitalium is a small parasitic bacterium that lives on the ciliated epithelial cells of the primate genital and respiratory tracts. M. genitalium is the smallest known genome that can constitute a cell, and the second-smallest bacterium after the recently-discovered endosymbiont...
, 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-formsL-form bacteria
L-form bacteria, also known as L-phase bacteria, L-phase variants, and cell wall-deficient bacteria, are strains of bacteria that lack cell walls...
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.
The first species of Mycoplasma/Mollicutes, that could be isolated was Mycoplasma mycoides
Mycoplasma mycoides
Mycoplasma mycoides is a bacterial species of the genus Mycoplasma in the class Mollicutes.This microorganism is a parasite that lives in ruminants , causing lung disease....
. This bacterium was cultivated
Microbiological culture
A microbiological culture, or microbial culture, is a method of multiplying microbial organisms by letting them reproduce in predetermined culture media under controlled laboratory conditions. Microbial cultures are used to determine the type of organism, its abundance in the sample being tested,...
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 (prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s, in 1956 called "Schizomycetes") or to the eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s. As type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the 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
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
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 morphologyMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
(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 peptidoglycanPeptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
; - 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.
Modern taxonomy
The present taxonomy of mollicutes is based on: 1. molecular analysis, especially of 16S rRNA16S ribosomal RNA
16S ribosomal RNA is a component of the 30S subunit of prokaryotic ribosomes. It is approximately 1.5kb in length...
; 2. serology (reactions on antiserum) and 3. phenotype (morphology, requirements for grow).
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 taxonomyLinnaean taxonomy
Linnaean taxonomy can mean either of two related concepts:# the particular form of biological classification set up by Carl Linnaeus, as set forth in his Systema Naturæ and subsequent works...
) or according to ancestry/genetic origin (cladistics
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
). In taxonomy, bacteria are preferably classified according to their descent.
Although the group Mollicutes phenotypically
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
(without a cell wall and therefore resistant to antibiotics like penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
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 Thermoplasma
Thermoplasma
In taxonomy, Thermoplasma is a genus of the Thermoplasmataceae.Thermoplasma is a genus of archaea. It belongs to the Thermoplasmata, which thrive in acidic and high-temperature environments. Thermoplasma are facultative anaerobes and respire using sulfur and organic carbon...
s 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
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
, elongation factor
Elongation factor
Elongation factors are a set of proteins that facilitate the events of translational elongation, the steps in protein synthesis from the formation of the first peptide bond to the formation of the last one.Elongation is the most rapid step in translation:...
or another protein.
Moreover cladistic analyses are often based on assumptions that are hidden in the used computational algorithms
Computational phylogenetics
Computational phylogenetics is the application of computational algorithms, methods and programs to phylogenetic analyses. The goal is to assemble a phylogenetic tree representing a hypothesis about the evolutionary ancestry of a set of genes, species, or other taxa...
, 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
Firmicutes
The Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...
, whereas in the announced 2nd edition, vol. 3 they are moved to a separate phylum Tenericutes
Tenericutes
Tenericutes is a phylum of bacteria that contains the Class Mollicutes. The name was validated in 1984 as a new division ....
.
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".
The phylum Tenericutes was already validly published in 1984 by R.G.E. Murray.
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.