User:Jkim777/Myxococcus xanthus

From Wikipedia, the free encyclopedia Myxococcus xanthus is a gram-negative, bacillus (or rod-shaped) species of myxobacteria that typically found in the top-most layer of soil. This bacteria lacks flagella; rather it uses pili for motility purposes. M. xanthus is well-known for its predatory behavior on other microorganisms. This bacteria sources carbon from lipids rather than sugars. It exhibits various forms of self-organizing behavior in response to environmental cues. Under normal conditions with abundant food, it exists as a predatory, saprophytic single-species biofilm called a swarm, highlighting the importance of intercellular communication for this bacteria. Under starvation conditions, it undergoes a multicellular development cycle.

Morphology
M. xanthus appear as gram-negative rods without flagella. These rods have an average length of 7 microns and width of 0.5 microns. It utilizes type IV pilus (T4P) to move in a "gliding" manner, crawling along a surface. As a colony or swarm, M. xanthus appear as a thin layer of ripples, often moving toward prey. In its spore form, the bacterium become a sphere with a thick outer membrane. This spore is yellow-orange, giving M. xanthus its name (xanthós, Ancient Greek meaning "golden").

Environment
M. xanthus is typically found in the top most layer of soil, preying as a "pack" on other microorganism like bacteria or fungi. It is a neutralophile, growing best between a pH of 7.2-8.2. The bacteria is characterized as a mesophile, growing best within the temperature range of 34-36°C. Like other Myxococcus bacteria, it is an obligate aerobe.

Metabolism
M. xanthus is a chemoorganoheterotroph. It obtains energy from oxidation-reduction reactions, uses organic molecules as a source of electrons, and sources carbon from organic molecules. This bacteria does produce and consume glycogen, a branched glucose polymer, but cannot fully convert glucose to pyruvate though the Embden-Meyerhof-Parnas pathway. The flux through the pathway is incomplete, even though homologs of each enzyme are present in the genome. Because of this reason, M. xanthus cannot rely on sugars for growth. It is hypothesized that the incomplete glycolytic pathway produce substrates for lipid metabolism.

Instead, M. xanthus relies on lipid metabolism to source carbon. The bacteria demonstrates a diverse set of lipid reactions, especially in lipid anabolism. Phospholipids are broken down into the head group, glycerol, and the two fatty acids. The fatty acids are degraded through β-oxidation at the carboxyl end of the fatty acid. M. xanthus expresses a wide variety of fatty acids. The calls contain at least 18 different fatty acids compared to the 3 to 5 fatty acids seen in most Proteobacteria. Redundancy in the fatty acid elongation enzymes and desaturase enzymes may contribute to this diversity of fatty acids. It is also noted that M. xanthus produces ether lipids, typically seen in eukaryotes.

To produce nucleic acids, M. xanthus salvages purines and pyrimidines from its prey. Amino acids are treated similarly, with the majority undergoing further catalysis for use in other pathways as needed.

Evolution
In 2003, two scientists, Velicer and Yu, deleted certain parts of the M. xanthus genome, making it unable to swarm effectively on soft agar. Individuals were cloned, and allowed to evolve. After a period of 64 weeks, two of the evolving populations had started to swarm outward almost as effectively as normal wild-type colonies. However, the patterns of the swarm were very different from those of the wild-type bacteria. This suggested that they had developed a new way of moving, and Velicer and Yu confirmed this by showing that the new populations had not regained the ability to make pili, which allows wild-type bacteria to swarm. This study addressed questions about the evolution of cooperation between individuals that had plagued scientists for years.

The evolution of M. xanthus can largely be attributed by two mechanisms of gene transfer such as LGT and vertical gene transfer. In this myxobacteria LGT suggests aquistiion of genes has come from other species of bacteria and this is also supported with the fact that the trait of M. xanthus' fruiting body is not possible without alien genes. Very little is known about the evolutionary mechanisms present in M. xanthus. However, it has been discovered that it can establish a generalist predator relationship with different prey, among which is Escherichia coli. In this predator-prey relationship, a parallel evolution of both species is observed through genomic and phenotypic modifications, producing in subsequent generations a better adaptation of one of the species that is counteracted by the evolution of the other, following a co-evolutionary model known Red Queen hypothesis. However, the evolutionary mechanisms present in M. xanthus that produce this parallel evolution are still unknown.

Genetics
The genome of M. xanthus consists of one circular chromosome with one origin of replication, and no plasmids. In 2001, the genome of strain DK 1622 was determined to have 9.14 Mb. The genome size is considerably larger than other Proteobacteria, likely due to lineage-specific gene duplication. Over 90% of the genome contains genes that encode for proteins. (link to 46 from article)

In 2023, the R31 isolate of M. xanthus underwent whole genome sequencing amounting to 9.25Mb. The R31 isolate’s genome codes for roughly 55% core proteins, 25% accessory proteins, 13% specific proteins, and 10% proteins that are specific to the isolate. Strain-specific genes likely relate to the evolutionary and predatory aspects that are not found in other strains.

Strains

 * Myxococcus xanthus DK 1622
 * Myxococcus xanthus DZ2
 * Myxococcus xanthus DZF1
 * Myxococcus xanthus NewJersey2
 * Myxococcus xanthus DSM16526T

Whole genome comparisons have indicated that M. virescens is the same species as M. xanthus. M. virescens was first described in 1892, so has precedence.

Colony growth
A swarm of M. xanthus is a distributed system, containing millions of bacteria that communicate among themselves in a non-centralized fashion. Simple patterns of cooperative behavior among the members of the colony combine to generate complex group behaviors in a process known as "stigmergy". For example, the tendency for one cell to glide only when in direct contact with another results in the colony forming swarms called "wolf-packs" that may measure up to several inches wide. This behavior is advantageous to the members of the swarm, as it increases the concentration of extracellular digestive enzymes secreted by the bacteria, thus facilitating predatory feeding. M. xanthus feeds on dead biomass of a broad range of bacteria and some fungi, discriminating living cells from dead cells, and causing cell death and lysis when required.

During stressful conditions, the bacteria undergo a process in which about 100,000 individual cells aggregate to form a structure called the fruiting body over the course of approximately twenty-four hours. The start of this process involves the cells displaying low motility. After several hours, the cells suddenly undergo an fast period of motion in which cells form "streams" to increase cell density and begin forming layers to develop the fruiting body. On the interior of the fruiting body, the rod-shaped cells are differentiated into spherical, thick-walled spores. They undergo changes in the synthesis of new proteins, as well as alterations in the cell wall, which parallel the morphological changes. During these aggregations, dense ridges of cells move in ripples, which wax and wane over 5 hours.

Motility
M. xanthus exhibit two main types of motility, known as A-motility and S-motility. A-motility (adventurous), otherwise known as "gliding", is a method of locomotion that allows for forward movement on single cells, without the help of flagella, on a solid surface. There are more than 37 genes are involved in the A-motility system. This form of motility is facilitated by Glt complexes in the cell envelope of the cell, which is powered using a molecular motor called an Agl. The molecular motors in M. xanthus is driven by the H+ ion gradient. Each bacterial cell has an array of motors along the cell body, which are localized to the periplasmic space in the cell envelope, but bound to the peptidoglycan layer in the cell wall. The motors are hypothesized to move on helical cytoskeletal filaments.

The combination of the Glt complexes with the Agl motor allows for focal adhesion and move freely in the outer membrane, and provide contact with the substratum. Extracellular polysaccharide slime assists with the gliding movement across a surface. This bacteria is limited to forward movement, and contains a lagging pole on the end which opposes the motion.

M. xanthus have the ability to use a second type of motility. This motility is called Social motility (S-motility), in which single cells do not move, but rather cells that are closer together move. This leads to a spatial distribution of cells with many clusters and few isolated single cells. This motility depend on the presence of the Type IV pili and diverse polysaccharides.

S-motility may represent a variation of twitching motility, since it is mediated by the extension and retraction of type IV pili that extend through the leading cell pole. The genes of the S-motility system appear to be homologs of genes involved in the biosynthesis, assembly, and function of twitching motility in other bacteria.

Cell differentiation, fruiting and sporulation
In response to starvation, since myxobacteria are neither chemolithotrophs or autotrophs, they direct their resources to develop species-specific multicellular fruiting bodies that are capable in aiding in social cooperation for predation. Starting from a uniform swarm of cells, some aggregate into fruiting bodies, while other cells remain in a vegetative state. Those cells that participate in formation of the fruiting body transform from rods into spherical, heat-resistant myxospores, while the peripheral cells remain rod-shaped. Although they are not as tolerant to environmental extremes as Bacillus endospores, the relative resistance of myxospores to desiccation and freezing enables myxobacteria to survive seasonally harsh environments. When a nutrient source becomes once again available, the myxospores germinate, shedding their spore coats to emerge into rod-shaped vegetative cells. The synchronized germination of thousands of myxospores from a single fruiting body enables the members of the new colony of myxobacteria to immediately engage in cooperative feeding.

M. xanthus cells can also differentiate into environmentally-resistant spores in a starvation-independent manner. This process, known as chemically induced sporulation, is triggered by the presence of glycerol and other chemical compounds at high concentrations. The biological implications of this sporulation process have been controversial for decades due to the unlikeliness to find such high concentrations of chemical inducers in their natural environment. However, the finding that the antifungal compound ambruticin acts as a potent natural inducer at concentrations expected to be present in soil, suggests that chemically induced sporulation is the result of competition and communication with the ambruticin-producing myxobacterium Sorangium cellulosum.

Intercellular communication
It is very likely that cells communicate during the process of fruiting and sporulation, because a group of cells that starved together form myxospores inside fruiting bodies. Intercellular signal appears to be necessary to ensure that sporulation happens in the proper place and at the proper time. Research supports the existence of an extracellular signal, A-factor, which is necessary for developmental gene expression and for the development of a complete fruiting body. This signaling mechanism additionally is capable of measuring the size of the surrounding aggregates.

Ability to eavesdrop
It has been shown that an M. xanthus swarm is capable of eavesdropping on the extracellular signals that are produced by the bacteria it preys upon, leading to changes in swarm behavior and increasing its efficiency as a predator. In the presence of acyl homoserine lactones, which are the signals produced by prey intended for other prey, M. xanthus transforms toward more vegetative predatory cells instead of myxospores. This allows for a highly adaptive physiology that will have likely contributed to the near ubiquitous distribution of the myxobacteria. This bacteria also responds to a chemoattractant called phosphatidyl ethanolamine, which is expelled when the prey dies. The chemoattractant draws in more M. xanthus, allowing for total lysis of prey cells. In order for M. xanthus to eavesdrop, there needs to be a high concentration of signals emitting between prey, which can occur when phosphatidyl ethanolamine is released, attracting more prey.

Developmental cheating
Social cheating exists among M. xanthus commonly. As long as mutants are not too common, if they are unable to perform the group beneficial function of producing spores, they will still reap the benefit of the population as a whole. Research has shown that 4 different types of M. xanthus mutants showed forms of cheating during development, by being over-represented among spores relative to their initial frequency in the mixture.

Importance in research
The complex life cycles of the myxobacteria make them very attractive models for the study of gene regulation as well as cell to cell interactions. The traits of M. xanthus make it very easy to study, and therefore important to research. Laboratory strains of M. xanthus are available that are capable of planktonic growth in shaker culture, so that they are easy to grow in large numbers. The tools of classical and molecular genetics are relatively well-developed in M. xanthus.

Although the fruiting bodies of M. xanthus are relatively primitive compared with, say, the elaborate structures produced by Stigmatella aurantiaca and other myxobacteria, the great majority of genes known to be involved in development are conserved across species. In order to make agar cultures of M. xanthus grow into fruiting bodies, one simply can plate the bacteria on starvation media. Furthermore, it is possible to artificially induce the production of myxospores without the intervening formation of fruiting bodies, by adding compounds such as glycerol or various metabolites to the medium. In this way, different stages in the developmental cycle can be experimentally isolated.

Videos

 * Myxococcus xanthus preying on an E. coli colony
 * Myxococcus xanthus fruiting body formation
 * Myxococcus xanthus ripples – Predation
 * Predatory bacterial crowdsourcing