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The name “Mycoplasma” is often used to describe bacteria of the Class Mollicutes, but more specifically refers to one of its nine genera, “mycoplasma”, which itself comprises more than 100 species. Other Mollicutes genera include Acholeplasma, Anaeroplasma, Asteroleplasma, Entomoplasma, Mesoplasma, Spiroplasma, Ureaplasma, and Candidatus Phytoplasma.

A recurring burden for cell cultures

Mycoplasmas are well known as cell-culture contaminants, with an estimated 15% plus of all cell lines being contaminated. These unwanted guests often interfere with various cell-culture parameters and this may in turn invalidate or compromise research outcomes (ie alteration of virus titers, gene expression, and cell metabolism). Although a number of detection assays are now reliable for detecting these microbial contaminations, efforts dedicated to eradicating mycoplasmas from cell cultures have turned out to be challenging. Therefore, these nuisances are still a main concern for any field of research or for biotechnological companies dealing with cell cultures or related biological products.

A major concern for public and animal health

The role of mycoplasmas as infectious agents has long been underestimated, mainly because rapid, specific diagnostic procedures were not available. It is now clearly demonstrated that several Mycoplasma and Ureaplasma species are significant pathogens of human and animal M. pneumoniae, which causes upper and lower respiratory tract infections in humans and is now also implicated in playing a significant a role in exacerbation of asthma in children (Waites et al. 2009). The picture is also changing for other species, such as Mycoplasma genitalium, which was under-diagnosed and is now recognised as a common cause of urethritis. A relatively large number of mycoplasma species found in animals are responsible for clearly identifiable diseases that cause significant economic loss in livestock production worldwide. For example, several diseases induced by ruminant mycoplasma species (that is, M. agalactiae, M. mycoides subsp. mycoides SC) are listed by the OIE. Mycoplasma species generally display a strict host and tissue specificity with a predilection for the respiratory and the urogenital tracts, the mammary gland, the joints and the serous membranes. With rare exceptions, the mechanisms and virulence factors implicated in mycoplasma pathogenesis remain to be elucidated. For most of these mycoplasmoses, efficient preventive or curative control methods have yet to be developed, and, as for other bacteria, the frequency of antibiotic resistance is increasing (Bébéar et al. 2005).

Plant yellowing: ask your phytoplasmologist

Plant pathogenic mollicutes (phytoplasmas and spiroplasmas) are phloem-limited bacteria that are responsible for hundreds of diseases affecting a wide variety of economically important crops, such as ornamentals, vegetables, fruit trees and grapevines. These pathogens are transmitted by phloem sap-feeding insects and, with the exception of spiroplasmas, they cannot be multiplied in vitro. There is no curative method and cases of plant tolerance/ resistance to phytoplasma diseases are extremely limited. Control strategies include detection and eradication of infected plants, production and use of healthy plant material and, in few cases, chemical treatments against the vector insect. These chemical treatments of crops are expensive and deleterious to the environment. In 2004, the first sequencing of a phytoplasma genome opened new avenues for phytoplasma research and provided a new basis for understanding the complex interactions occurring with the plant hosts and with the insect vectors (Strauss E, 2009).

Living micro-transporters and biomotors

Bacteria have evolved different means for generating movement and Mollicutes motility has been shown to be of particular interest. Some of the mycoplasma species (that is, M. mobile, see below) are able to glide on inert surfaces and spiroplasmas propel themselves by a unique mechanism that involves two temporally distinct kinks that travel the length of the bacterium (Shaevitz 2005).

One mycoplasma species isolated from fish, Mycoplasma mobile, adheres to plastic surfaces and moves around vigorously at speeds of up to five micrometers per second. Because of these properties, it has been studied as a potential bio-motor by a Japanese group. Indeed, it was shown that this gliding motor uses the chemical energy of ATP and transforms it into mechanical work by acting at micro/nanotechnology scale. On patterned lithographic substrates, M. mobile cells move along the bottom edge of the walls so that its direction can be controlled depending on the pattern. Because cells loaded with streptavidin beads following biotinylation of surface proteins moved at normal speeds, it has been suggested that these bacteria could be useful as living microtransporters, carrying cargo around within micrometer-scale spaces (Hiratsuka et al. 2005). The group further engineered a hybrid micromachine by docking a 20-μm-diameter silicon dioxide rotor to the mycoplasmas via biotin-streptavidin interactions. As a result, the rotor is pulled by the mycoplasma cells which glide along silicon tracks and rotate (Hiratsuka et al. 2006). This combination includes the precise engineering of synthetic devices with efficient energy conversion and takes advantage of the potential for self-repair of such biological systems.

An alternative which would avoid potential biohazards is the use of M. mobile “ghosts”, which are not alive due to partial membrane dissolution, but still demonstrate gliding as shown by Uenoyama and Miyata in 2005. In the distant future, this system could be used to build micro-robots driven by biological motors that would be able to move around and do mechanical work in a “micro-world”.

A platform for a synthetic life

The Mycoplasma genus represents a unique category of bacteria that have been portrayed as “minimal” self-replicating organisms due to their limited genomic content. More specifically, mycoplasmas are characterised by a total lack of cell wall and genomes with a small size (0.58- 2.2 Mbp) and a low G+C content (23 to 40%). Because of these particular features, mycoplasmas are currently used as models to create artificial life in the laboratory by making the first synthetic bacterium (Craig Venter Institute). Recently, Venter’s team succeeded in producing the first artificial bacterial chromosome: using the genome sequence of Mycoplasma genitalium as a blueprint, they stitched together piece of synthesised DNA (Gibson et al. 2008). Assembly of the synthetic genome was made in yeast in which genetic engineering is easier. In the meantime, the same team transplanted the genome of Mycoplasma mycoides into the hollowed-out shell of a different mycoplasma species, Mycoplasma capricolum (Lartigue et al. 2007, 2009). The transplanted genome “booted up” and colonies of M. mycoides were produced. In the near future, the combination of these two breakthroughs, the synthesis of an artificial mycoplasma and its transplantation into an empty shell, may produce the first synthetic life, Mycoplasma laboratorium. This new life form might mark a new turn in synthetic genomics by offering a model for re-designing micro-organisms and a platform for producing all sorts of molecules.

Current and future challenges

In the 1990s, Mycoplasma genitalium was chosen as one of the few model organisms to validate the large-scale sequencing technologies that ultimately lead to sequencing the human genome. This organism and mycoplasmas in general are still considered as models to explore new technological approaches because of their minimal cell organisation and metabolism. Despite this apparent simplicity, mycoplasmas are more and more recognised as significant pathogens of plant, animals and humans, causing infections that remained under-diagnosed for many years and for which innovative control strategies remain to be developed.

Data gathered from the sequencing of several mycoplasma genomes have confirmed that mycoplasmas can be considered as among the most evolved bacteria while being at the same time among the simplest life form. The novelty is that this evolution was extremely dynamic (Sirand-Pugnet P et al. 2007) and benefited not only from a capacity to evolve faster than other cells but also from sharing genetic information among species colonising the same host. The ability to build synthetic genomes offers a unique opportunity to experimentally validate predictions from the strongly emerging field of systems biology. In the far future, this artificial life platform could be used to build micro-manufactory or micro-robots driven by biological motors, as well as to re-design pathogenic organisms into efficient vaccines.

For reviews on mycoplasma cell interactions, see Citti et al. 2005 and Rottem et al. 1998

For more information, contact :

Christine Citti, UMR1225, INRA, ENVT Ecole Nationale Vétérinaire de Toulouse 23 Chemin des Capelles, BP 87614 31076 Toulouse Cedex 3 France
E-mail: c.citti@envt.fr

Alain Blanchard, UMR1090, INRA, Université de Bordeaux 71 Avenue Edouard Bourleaux, BP 81 33883 Villenave d’Ornon, France
E-mail: ablancha@bordeaux.inra.fr

Websites of interest International Organization for Mycoplasmology http://the-iom.org/

A database dedicated to the comparative genomics of Mollicutes
Website: www.cbi.labri.fr/outils/molligen/