Myxococcus xanthus
Encyclopedia
Myxococcus xanthus colonies exist as a self-organized, predatory
, saprotrophic, single-species biofilm
called a swarm. Myxococcus xanthus, which can be found almost ubiquitously in soil, are thin rod shaped
, gram-negative
cells that exhibit self-organizing behavior as a response to environmental cues. The swarm, which has been compared to a "wolf-pack,"
modifies its environment through stigmergy
. This behavior facilitates predatory feeding, as the concentration of extracellular digestive enzymes secreted by the bacteria increases. M. xanthus is a model organism
for studying development, the behavior in which starving bacteria self-organize to form fruiting bodies: dome shaped structures of approximately 100,000 cells. These swarms differentiate into metabolically quiescent and environmentally resistant myxospores over the course of several days. During this process of self-organizing, dense ridges of cells move in traveling waves (ripples) that grow and shrink over several hours.
A swarm of M. xanthus is a distributed system: a population of millions of identical entities that communicate among themselves in a non-centralized fashion, thus behaving as a single entity. The cells within the swarm form a collective, exhibiting coordinated movement through a series of signals to create dynamic patterns in response to environmental cues. One of these behaviors, development (mentioned above), is controlled through a cascade, or series, of transcriptional
regulators (TR) that control downstream gene expression. It has been proposed that all emergent
, or self-organizing, behavior in M. xanthus is under this type of control.
, the largest prokaryotic genome
sequenced until the sequencing of Sorangium cellulosum
(12.3 Mb). The M. xanthus genome
was sequenced by Monsanto
Company and The Institute for Genomic Research (TIGR
) and is currently available in GenBank
.
s, mold
s, and enterobacteria. M. xanthus is also one of the most genetically tractable myxobacteria
, and there is a considerable body of ongoing research aimed at genetically modifying M. xanthus to overproduce these compounds in a controlled environment, such as a fermentor. Current work includes the heterologous production of Epothilone
B, a polyketide
capable of targeting breast cancer cells that is superior to the anti-tumor drug Taxol. Finally, M. xanthus offers potential utility in the agricultural arena. Published data indicate that M. xanthus may act as a bacterial biological control agent to inhibit pathogenic fungi in plants.
known as twitching motility. Twitching is mediated by a Type IV pilus that extrudes through the pole of the cell. The pilus is extended by the addition of pilin monomers at the base of the pilus. The tip of the pilus binding to a substrate in the biofilm
sends a signal for the pilus to retract by the removal of pilin subunits. The shortening of the pilus leads to the cell being dragged along the surface towards the tip of the pilus. The mechanism of the Type IV pilus has been likened to a grappling hook
. The pilus apparatus is only constructed at one pole at a given time, but the location is dynamic and dependent upon chemotactic
signals.
In addition to Type IV pilus mediated group motility, M. xanthus cells also move individually using gliding motility on solid surfaces. Cells move in one direction along their long axis, then pause and continue movement in the opposite direction. The molecular mechanisms driving gliding motility are currently unknown.
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
, saprotrophic, single-species biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
called a swarm. Myxococcus xanthus, which can be found almost ubiquitously in soil, are thin rod shaped
Bacilli
Bacilli refers to a taxonomic class of bacteria. It includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens like Bacillus anthracis .-Ambiguity:...
, gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
cells that exhibit self-organizing behavior as a response to environmental cues. The swarm, which has been compared to a "wolf-pack,"
Pack (canine)
Pack is a social group of conspecific canids. Not all species of canids - notably the red fox - form packs. Pack size and social behaviour within packs varies across species.-Species which exhibit pack behavior:...
modifies its environment through stigmergy
Stigmergy
Stigmergy is a mechanism of indirect coordination between agents or actions. The principle is that the trace left in the environment by an action stimulates the performance of a next action, by the same or a different agent...
. This behavior facilitates predatory feeding, as the concentration of extracellular digestive enzymes secreted by the bacteria increases. M. xanthus is a model organism
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
for studying development, the behavior in which starving bacteria self-organize to form fruiting bodies: dome shaped structures of approximately 100,000 cells. These swarms differentiate into metabolically quiescent and environmentally resistant myxospores over the course of several days. During this process of self-organizing, dense ridges of cells move in traveling waves (ripples) that grow and shrink over several hours.
A swarm of M. xanthus is a distributed system: a population of millions of identical entities that communicate among themselves in a non-centralized fashion, thus behaving as a single entity. The cells within the swarm form a collective, exhibiting coordinated movement through a series of signals to create dynamic patterns in response to environmental cues. One of these behaviors, development (mentioned above), is controlled through a cascade, or series, of transcriptional
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
regulators (TR) that control downstream gene expression. It has been proposed that all emergent
Emergent
It may also mean:* Emergent , Neural Simulation Software* Emergent , a 2003 album by Gordian Knot* emergent plant, a plant which grows in water but which pierces the surface so that it is partially in air...
, or self-organizing, behavior in M. xanthus is under this type of control.
Genomics
The complexity of the M. xanthus life cycle is reflected in its 9.14 Megabase genomeGenome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
, the largest prokaryotic genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
sequenced until the sequencing of Sorangium cellulosum
Sorangium cellulosum
Sorangium cellulosum is a soil-dwelling Gram-negative bacterium of the group myxobacteria. It is motile and shows gliding motility. It has an unusually-large genome of 13,033,779 base pairs, making it the largest bacterial genome sequenced to date....
(12.3 Mb). The M. xanthus genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
was sequenced by Monsanto
Monsanto
The Monsanto Company is a US-based multinational agricultural biotechnology corporation. It is the world's leading producer of the herbicide glyphosate, marketed in the "Roundup" brand of herbicides, and in other brands...
Company and The Institute for Genomic Research (TIGR
TIGR
TIGR, abbreviation for Trst , Istra , Gorica and Reka , with the full name Revolutionary Organization of the Julian March T.I.G.R. was a militant anti-Fascist and insurgent organization active in the 1920s and the 1930s in the eastern Italian border region known as the Julian March.The...
) and is currently available in GenBank
GenBank
The GenBank sequence database is an open access, annotated collection of all publicly available nucleotide sequences and their protein translations. This database is produced and maintained by the National Center for Biotechnology Information as part of the International Nucleotide Sequence...
.
Medical relevance
Beyond its use as a model for self-organization, M. xanthus also produces hundreds of potentially valuable secondary metabolites. During predation, M. xanthus releases over 300 secondary metabolites, many of which are used to lyse the cells of soil microbes on which it feeds. Many of these secondary metabolites are known to have medicinal properties. For example, M. xanthus produces myxalamid, an antibiotic that targets yeastYeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
s, mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...
s, and enterobacteria. M. xanthus is also one of the most genetically tractable myxobacteria
Myxobacteria
The myxobacteria are a group of bacteria that predominantly live in the soil. The myxobacteria have very large genomes, relative to other bacteria, e.g. 9-10 million nucleotides. Sorangium cellulosum has the largest known bacterial genome, at 13.0 million nucleotides...
, and there is a considerable body of ongoing research aimed at genetically modifying M. xanthus to overproduce these compounds in a controlled environment, such as a fermentor. Current work includes the heterologous production of Epothilone
Epothilone
The epothilones are a new class of cancer drugs. Like taxanes, they prevent cancer cells from dividing by interfering with tubulin, but in early trials epithilones have better efficacy and milder adverse effects than taxanes....
B, a polyketide
Polyketide
Polyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
capable of targeting breast cancer cells that is superior to the anti-tumor drug Taxol. Finally, M. xanthus offers potential utility in the agricultural arena. Published data indicate that M. xanthus may act as a bacterial biological control agent to inhibit pathogenic fungi in plants.
Motility
M. xanthus cells exhibit a type of surface-mediated motilityMotility
Motility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...
known as twitching motility. Twitching is mediated by a Type IV pilus that extrudes through the pole of the cell. The pilus is extended by the addition of pilin monomers at the base of the pilus. The tip of the pilus binding to a substrate in the biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
sends a signal for the pilus to retract by the removal of pilin subunits. The shortening of the pilus leads to the cell being dragged along the surface towards the tip of the pilus. The mechanism of the Type IV pilus has been likened to a grappling hook
Grappling hook
A grappling hook is an anchor with multiple hooks , attached to a rope; it is thrown, dropped, sunk, projected, or fastened directly by hand to where at least one hook may catch and hold. Generally, grappling hooks are used to temporarily secure one end of a rope. They may also be used to dredge...
. The pilus apparatus is only constructed at one pole at a given time, but the location is dynamic and dependent upon chemotactic
Chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
signals.
In addition to Type IV pilus mediated group motility, M. xanthus cells also move individually using gliding motility on solid surfaces. Cells move in one direction along their long axis, then pause and continue movement in the opposite direction. The molecular mechanisms driving gliding motility are currently unknown.