Mycoplasma

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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, 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.

Origin of the name
The name Mycoplasma, from the Greek mykes (fungus) and plasma (formed), was first used by A. B. Frank in 1889. He thought it was a fungus, due to fungus-like characteristics .

An older name for Mycoplasma was Pleuropneumonia-Like Organisms (PPLO), referring to organisms similar to the causative agent of contagious bovine pleuropneumonia (CBPP). It was later found that the fungus-like growth pattern of M. mycoides is unique to that species.

Characteristics
There are over 100 recognized species of the genus Mycoplasma, one of several genera within the bacterial class Mollicutes. Mollicutes are parasites or commensals of humans, other animals (including insects), and plants; the genus Mycoplasma is by definition restricted to vertebrate hosts. Cholesterol is required for the growth of species of the genus Mycoplasma as well as certain other genera of mollicutes. Their optimum growth temperature is often the temperature of their host if warmbodied (e. g. 37° C in humans) or ambient temperature if the host is unable to regulate its own internal temperature. Analysis of 16S ribosomal RNA sequences as well as gene content strongly suggest that the mollicutes, including the mycoplasmas, are closely related to either the Lactobacillus or the Clostridium branch of the phylogenetic tree (Firmicutes sensu stricto).

Cell morphology
The bacteria of the genus Mycoplasma (trivial name: mycoplasmas) and their close relatives are characterized by lack of a cell wall. Despite this, the cells often present a certain shape, with a characteristic small size, with typically about 10% of the volume of an Escherichia coli cell. These cell shapes presumably contribute to the ability of mycoplasmas to thrive in their respective environments. Most are pseudococcoidal, but there are notable exceptions. Species of the M. fastidiosum cluster are rod-shaped. Species of the M. pneumoniae cluster, including M. pneumoniae, possess a polar extension protruding from the pseudococcoidal cell body. This tip structure, designated an attachment organelle or terminal organelle, is essential for adherence to host cells and for movement along solid surfaces (gliding motility), and is implicated in normal cell division. M. pneumoniae cells are pleomorphic, with an attachment organelle of regular dimensions at one pole and a trailing filament of variable length and uncertain function at the other end, whereas other species in the cluster typically lack the trailing filament. Other species like M. mobile and M. pulmonis have similar structures with similar functions.

Mycoplasmas are unusual among bacteria in that most require sterols for the stability of their cytoplasmic membrane. Sterols are acquired from the environment, usually as cholesterol from the animal host. Mycoplasmas generally possess a relatively small genome of 0.58-1.38 megabases, which results in drastically reduced biosynthetic capabilities and explains their dependence on a host. Additionally they use an alternate genetic code where the codon UGA is encoding for the amino acid tryptophan instead of the usual opal stop codon. They have a low GC-content (23-40 mol %).

First isolation
In 1898 Nocard and Roux reported the cultivation of the causative agent of CBPP, which was at that time a grave and widespread disease in cattle herds. The disease is caused by M. mycoides subsp. mycoides SC (small-colony type), and the work of Nocard and Roux represented the first isolation of a mycoplasma species. Cultivation was, and still is difficult because of the complex growth requirements.

These researchers succeeded by inoculating a semi-permeable pouch of sterile medium with pulmonary fluid from an infected animal and depositing this pouch intraperitoneally into a live rabbit. After fifteen to twenty days, the fluid inside of the recovered pouch was opaque, indicating the growth of a microorganism. Opacity of the fluid was not seen in the control. This turbid broth could then be used to inoculate a second and third round and subsequently introduced into a healthy animal, causing disease. However, this did not work if the material was heated, indicating a biological agent at work. Uninoculated media in the pouch, after removal from the rabbit, could be used to grow the organism in vitro, demonstrating the possibility of cell-free cultivation and ruling out viral causes, although this was not fully appreciated at the time (Nocard and Roux, 1890).

Small genome
Recent advances in molecular biology and genomics have brought the genetically simple mycoplasmas, particularly M. pneumoniae and its close relative M. genitalium, to a larger audience. The second published complete bacterial genome sequence was that of M. genitalium, which has one of the smallest genomes of free-living organisms. The M. pneumoniae genome sequence was published soon afterwards and was the first genome sequence determined by primer walking of a cosmid library instead of the whole-genome shotgun method. Mycoplasma genomics and proteomics continue in efforts to understand the so-called minimal cell, catalog the entire protein content of a cell, and generally continue to take advantage of the small genome of these organisms to understand broad biological concepts.

Taxonomy
The medical and agricultural importance of members of the genus Mycoplasma and related genera has led to the extensive cataloging of many of these organisms by culture, serology, and small subunit rRNA gene and whole genome sequencing. A recent focus in the sub-discipline of molecular phylogenetics has both clarified and confused certain aspects of the organization of the class Mollicutes.

Originally the trivial name "mycoplasmas" has commonly denoted all members of the class Mollicutes. The name "Mollicutes" is derived from the Latin mollis (soft) and cutes (skin), and all of these bacteria do lack a cell wall and the genetic capability to synthesize peptidoglycan. Now Mycoplasma is a genus in Mollicutes. Despite the lack of a cell wall, many taxonomists have classified Mycoplasma and relatives in the phylum Firmicutes, consisting of low G+C Gram-positive bacteria such as Clostridium, Lactobacillus, and Streptococcus based on 16S rRNA gene analysis. The order Mycoplasmatales contains a single family, Mycoplasmataceae, comprising two genera: Mycoplasma and Ureaplasma.

Historically, the description of a bacterium lacking a cell wall was sufficient to classify it to the genus Mycoplasma and as such it is the oldest and largest genus of the class with about half of the class' species (107 validly described), each usually limited to a specific host and with many hosts harboring more than one species, some pathogenic and some commensal. In later studies, many of these species were found to be phylogenetically distributed among at least three separate orders.

A limiting criterion for inclusion within the genus Mycoplasma is that the organism have a vertebrate host. In fact, the type species, M. mycoides, along with other significant mycoplasma species like M. capricolum, is evolutionarily more closely related to the genus Spiroplasma in the order Entomoplasmatales than to the other members of the Mycoplasma genus. This and other discrepancies will likely remain unresolved because of the extreme confusion that change could engender among the medical and agricultural communities.

The remaining species in the genus Mycoplasma are divided into three non-taxonomic groups, hominis, pneumoniae and fermentans, based on 16S rRNA gene sequences. The hominis group contains the phylogenetic clusters of M. bovis, M. pulmonis, and M. hominis, among others. M. hyopneumoniae is a primary bacterial agent of the porcine respiratory disease complex.

The pneumoniae group contains the clusters of M. muris, M. fastidiosum, U. urealyticum, the currently unculturable haemotrophic mollicutes, informally referred to as haemoplasmas (recently transferred from the genera Haemobartonella and Eperythrozoon), and the M. pneumoniae cluster. This cluster contains the species (and the usual or likely host) M. alvi (bovine), M. amphoriforme (human), M. gallisepticum (avian), M. genitalium (human), M. imitans (avian), M. pirum (uncertain/human), M. testudinis (tortoises), and M. pneumoniae (human). Most if not all of these species share some otherwise unique characteristics including an attachment organelle, homologs of the M. pneumoniae cytadherence-accessory proteins, and specialized modifications of the cell division apparatus.

A study of 143 genes in 15 species of Mycoplasma suggests that the genus can be grouped into four clades: the M. hyopneumoniae group, the M. mycoides group, the M. pneumoniae group and a Bacillus-Phytoplasma group. The M. hyopneumoniae group is more closely related to the M. pneumoniae group than the M. mycoides group.

Laboratory contaminant
Mycoplasma species are often found in research laboratories as contaminants in cell culture. Mycoplasmal cell culture contamination occurs due to contamination from individuals or contaminated cell culture medium ingredients. Mycoplasma cells are physically small – less than 1 µm – and they are therefore difficult to detect with a conventional microscope. Mycoplasmas may induce cellular changes, including chromosome aberrations, changes in metabolism and cell growth. Severe Mycoplasma infections may destroy a cell line. Detection techniques include DNA Probe, enzyme immunoassays, PCR, plating on sensitive agar and staining with a DNA stain including DAPI or Hoechst.

It has been estimated that at least 11 to 15% of U.S. laboratory cells cultures are contaminated with mycoplasma. A Corning study showed that half of U.S. scientists did not test for mycoplasma contamination in their cell cultures. The study also stated that, in Czechoslovakia, 100% of cell cultures that were not routinely tested were contaminated while only 2% of those routinely tested were contaminated. (study page 6) Since the U.S. contamination rate was based on a study of companies that routinely checked for mycoplasma, the actual contamination rate may be higher. European contamination rates are higher and that of other countries are higher still (up to 80% of Japanese cell cultures). About 1% of published Gene Expression Omnibus data may have been compromised. (Link into RNAnet showing contamination of GEO. Press plot and drag blue crosshairs to expose links to description of experiments on human RNA samples). Several antibiotic based formulation of anti-mycoplasma reagents have been developed over the years, including BM-Cyclin by Roche, MRA by ICN, Plasmocin by Invivogen and more recently De-Plasma by TOKU-E.

First synthetic genome synthesized
On 20 May 2010, Gibson et al. reported that they had created the first synthetic genome of a mycoplasmal cell.