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.
Edward and Freundt 1967
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).
In 1962, R.G.E. Murray proposed to divide the kingdom Bacteria into three divisions (= phyla) on the basis of the cell wall types:
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).
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 (see Figure).
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. The results of molecular phylogenetic analyses have partly depend on the chosen molecular marker, like rRNA, elongation factor or another protein. Phylogenetic trees based on phosphoglycerate kinase (Pgk) amino acid sequences' indicated a monophyletic origin for the Mollicutes within the Firmicutes.
An early edition of Bergey’s Manual of Systematic Bacteriology placed class Mollicutes within phylum Firmicutes, whereas in the announced 2nd edition, 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". In the Taxonomic Outline of Bacteria and Archaea (TOBA Release 7.7), March 2007, the Mollicutes are a class in the phylum Firmicutes.
Acholeplasma are wall-less bacteria in the Mollicutes class. They include saprotrophic or pathogenic species. There are 15 recognised species. The G+C content is low, ranging from 26 - 36% (mol%). The genomes of Acholeplasma species range in size from 1.5 to 1.65 Mbp. Cholesterol is not required for growth. The species are found on animals, and some plants and insects. The optimum growth temperature is 30 to 37 degrees Celsius.Acholeplasma laidlawii is a common contaminant of cell culture media products, and has also been used in extensive studies of lipid polymorphism because this organism alters its ratio of MGlcDG (monoglucosyl diacylglycerol) to DGlcDG (diglucosyl diacylglycerol) in response to growth conditions.Acholeplasmataceae
The Acholeplasmatales are an order in the class Mollicutes, containing only one family, Acholeplasmataceae, comprising the genera Acholeplasma and Phytoplasma. Yet, Phytoplasma has the candidatus state, because members still could not be cultured.
Etymology: The name Acholeplasmatales is derived from the Greek a = not, cholè = bile and plasma = anything moulded or formed.
Species in the order Acholeplasmatales can grow in a medium without cholesterol, unlike species in the order Mycoplasmatales. Cholesterol, a sterol, is an important component of the cell membrane of mycoplasmas, whereas in acholeplasmas and in bacteria in general it is absent.Anaeroplasmatales
Anaeroplasmatales is an order of mollicute bacteria which are generally found in the rumens of cattle and sheep. The only family in the order is the family Anaeroplasmataceae.Bacilli
Bacilli is a taxonomic class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as Bacillus anthracis (the cause of anthrax). Bacilli are almost exclusively gram-positive bacteria.Cell envelope
The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In gram-negative bacteria an outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is absent.
Bacterial cell envelopes fall into two major categories: a gram-positive type and a gram-negative type, distinguished by Gram staining. Either type may have an enclosing capsule of polysaccharide for extra protection. As a group these are known as polysaccharide encapsulated bacteria.Entomoplasmatales
Entomoplasmatales is a small order of mollicute bacteria.
The genus Spiroplasma is part of this order.Erysipelothrix rhusiopathiae
Erysipelothrix rhusiopathiae is a Gram-positive, catalase-negative, rod-shaped, non-spore-forming, nonacid-fast, nonmotile bacterium. Distributed worldwide, E. rhusiopathiae is primarily considered an animal pathogen, causing the disease known as erysipelas that may affect a wide range of animals. Pigs, turkeys and laying hens are most commonly affected, but cases have been reported in other mammals, birds, fish, and reptiles. In pigs, the disease is known as "diamond skin disease". The bacterium can also cause zoonotic infections in humans, called erysipeloid. The human disease called erysipelas is not caused by E. rhusiopathiae, but by various members of the genus Streptococcus.Firmicutes
The Firmicutes (Latin: firmus, strong, and cutis, skin, referring to the cell wall) 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. Scientists once classified the Firmicutes to include all gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus).
Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through anoxygenic photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage.Gracilicutes
Gracilicutes (Latin: gracilis, slender, and cutis, skin, referring to the cell wall) is a controversial taxon in bacterial taxonomy.
Traditionally gram staining results were most commonly used as a classification tool, consequently until the advent of molecular phylogeny, the Kingdom Monera (as the domains Bacteria and Archaea were known then) was divided into four phyla,
Gracilicutes (gram-negative, it is split in many groups, but some authors still use it in a narrower sense)
Firmacutes [sic] (gram-positive, subsequently corrected to Firmicutes, today it includes the Mollicutes but excludes the Actinobacteria)
Mollicutes (gram variable, posteriorly renamed as Tenericutes, e.g. Mycoplasma)
Mendosicutes (uneven gram stain, "methanogenic bacteria" now known as methanogens and classed as Archaea)This classification system was abandoned in favour of the three-domain system based on molecular phylogeny started by C. Woese.This taxon was revived in 2006 by Cavalier-Smith as an infrakindgom containing the phyla Spirochaetae, Sphingobacteria (FCB), Planctobacteria (PVC), and Proteobacteria. It is a gram-negative clade that branched off from other bacteria just before the evolutionary loss of the outer membrane or capsule, and just after the evolution of flagella. This taxon is not generally accepted and the three-domain system is followed.An almost identical taxon called Hydrobacteria was defined in 2009 using bioinfomatic methods. It is in contrast to the other major group of eubacteria called Terrabacteria. The analysis also altered the internal phylogeny of this group, with PVC-FCB-Spirochaetae placed forming a clade next to Proteobacteria. What was PVC is described as polyphyletic.Haloplasma
Haloplasma contractile is a halophilic, cell wall-less bacterium. It is the only known representative of a deep lineage, and is classified in its own family (Haloplasmataceae) and order (Haloplasmatales), in the class Mollicutes.International Organization for Mycoplasmology
The International Organisation for Mycoplasmology (IOM) is a non-profit making organisation founded in 1976. It promotes the study of mycoplasmas (mollicutes), bacteria without a cell wall, and the diseases associated with them.Mesoplasma
Mesoplasma is a genus of bacteria belonging to the class Mollicutes. Mesoplasma is related to the genus Mycoplasma but differ in several respects.Mycoplasma
Mycoplasma is a mollicute genus of bacteria that lack a cell wall around their cell membranes. This characteristic makes them naturally resistant to 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, including M. pneumoniae, which is an important cause of atypical pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. Mycoplasma species are the smallest bacterial cells yet discovered, can survive without oxygen, and come in various shapes. For example, M. genitalium is flask-shaped (about 300 x 600 nm), while M. pneumoniae is more elongated (about 100 x 1000 nm). Hundreds of mycoplasma species infect animals.Mycoplasmataceae
Mycoplasmataceae is a family of bacteria in the order Mycoplasmatales. This family consists of the genera Mycoplasma and Ureaplasma.
In 1967, the order Mycoplasmatales was incorporated into the class Mollicutes. Many species are sexually transmitted and cause pelvic inflammatory disease.Phytoplasma
Phytoplasmas are obligate bacterial parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-like organisms. Since their discovery, phytoplasmas have resisted all attempts at in vitro culture in any cell-free medium; routine cultivation in an artificial medium thus remains a major challenge. Although phytoplasmas have recently been reported to be grown in a specific artificial medium, experimental repetition has yet to be reported. Phytoplasmas are characterized by the lack of a cell wall, a pleiomorphic or filamentous shape, a diameter normally less than 1 μm, and a very small genome.
Phytoplasmas are pathogens of agriculturally important plants, including coconut, sugarcane, and sandalwood, in which they cause a wide variety of symptoms ranging from mild yellowing to death. Phytoplasmas are most prevalent in tropical and subtropical regions. They are transmitted from plant to plant by vectors (normally sap-sucking insects such as leafhoppers) in which they both survive and replicate.Plasmaviridae
Plasmaviridae is a family of bacteria-infecting viruses. Acholeplasma species serve as natural hosts. There are currently only one genus (Plasmavirus), and one species in this family: the type species Acholeplasma virus L2. All viruses known in this family have been isolated from species in the class Mollicutes.
This family is poorly studied and little is known about the diversity and biology of these viruses.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 and moves in a corkscrew motion. Many Spiroplasma are found either in the gut or haemolymph of insects where they can act to manipulate host reproduction, or defend the host as endosymbionts. Spiroplasma are also disease-causing agents in the phloem of plants. Spiroplasmas are fastidious organisms, which require a rich culture medium. Typically they grow well at 30 °C, but not at 37 °C. A few species, notably Spiroplasma mirum, grow well at 37 °C (human body temperature), and cause cataracts and neurological damage in suckling mice. The best studied species of spiroplasmas are Spiroplasma poulsonii, a reproductive manipulator and defensive insect symbiont, Spiroplasma citri, the causative agent of citrus stubborn disease, and Spiroplasma kunkelii, the causative agent of corn stunt disease.Spiroplasma citri
Spiroplasma citri is a bacterium species and the causative agent of Citrus stubborn disease.Its genome has been partially sequenced.The restriction enzyme SciNI, with the cutting site 5' GCGC / 3' CGCG, can be found in S. citri.
Euscelis plebejus can be used as a vector of the bacterium to experimentally infect white clover (Trifolium repens).Tenericutes
Tenericutes (tener cutis: soft skin) is a phylum of bacteria that contains the class Mollicutes. The name was validated in 1984 as a new division (phylum). Notable genera include Mycoplasma, Spiroplasma, Ureaplasma, and Phytoplasma.