Myxococcus xanthus
| Myxococcus xanthus | ||||||||||||
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M. xanthus forms fruiting bodies approx. 50 times |
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| Scientific name | ||||||||||||
| Myxococcus xanthus | ||||||||||||
| Beebe , 1941 |
Myxococcus xanthus is an obligatory aerobic delta proteobacterium . The rod-shaped cells form yellow to yellow-brown colonies ( xanthus "yellow") and are glidingly mobile. The genome of the bacterium has been completely sequenced, it has a high GC content of 69% on average.
M. xanthus is a model organism . As a representative of the myxobacteria , with 9.2 million base pairs and 7,500 genes, it has one of the largest bacterial genomes known to date; it is about twice as large as that of the intestinal bacterium Escherichia coli and even larger than that of some fungi . Like most myxobacteria, M. xanthus has the ability to cooperate and has a complex development cycle that is followed when the environmental conditions deteriorate.
Vegetative way of life
The cells live in the soil, on decomposing organic matter or manure. However, they feed less saprotrophically , i.e. on dead organic matter, but mainly live on other microorganisms that overwhelm them as a swarm: other cells are killed and broken down by toxic compounds and enzymes released to the outside . Accordingly, M. xanthus is hardly able to use carbohydrates , such as various sugars, as a source of energy and carbon, but lives as a “carnivore” almost exclusively on proteins and peptides . The bacteria mostly move in a sliding manner - a skill that is also known from other bacteria, but the mechanism of which has not yet been adequately understood.
development
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When the food supply is exhausted, the cells begin a complicated development process. The bacteria first synchronize the type of movement and their direction. In addition, at this stage the bacteria no longer move smoothly, but mainly with the help of their pili , which connect the cells to one another. This creates long chains of hundreds or thousands of cells. Here one can observe a process known as rippling is called: The Myxococcus - colony is then traversed by waves from aggregating and drifting apart cells.
The bacteria send out biochemical signals that serve to coordinate and synchronize the development process. More and more chains unite and finally form aggregates of hundreds of thousands of cells. Such aggregates can initially dissolve again or even merge with one another. Finally , the cells inside these aggregates differentiate into spores with thick cell walls which, as inactive resting stages, are resistant to dehydration, UV radiation and other harmful environmental influences. Due to their low metabolism, they can survive long periods of hunger and remain viable for several years. The three-dimensional structures that the spores contain are called the fruiting bodies . While M. xanthus forms relatively simply structured, hemispherical fruiting bodies, related genera such as Stigmatella or Corallococcus are able to build up more complex, stalked, branched and often conspicuously pigmented structures that are easily visible to the naked eye. The entire development process ( stigmergy ) takes several days under ideal circumstances.
It is estimated that only 5 to 10 percent of all cells that take part in a development process can end it as a spore. The rest die or lyse , so they dissolve. Possibly they serve as food for the other cells and thus enable them to stop developing.
Evolution of cooperative behavior
It is not entirely certain how myxobacteria benefit from this complicated process. Other bacteria, for example those of the genus Bacillus , sporulate as a single cell without first uniting to form fruiting bodies. It is believed that the social way of life of the myxobacteria is one reason for the common spore formation: If a fruiting body containing thousands of spores gets into a habitat with favorable environmental conditions, a new swarm is created on the spot. This is immediately able to produce enzymes and toxic substances in high concentrations and to resume the hunting strategy of the myxobacteria. Single cells would take much longer to form swarms, and the compounds released would diffuse into the environment and lose their effectiveness.
Motility
Myxobacteria usually move smoothly. This form of locomotion is known as A-motility . "A" stands for adventurous , which means something like "adventurous" or "enterprising". During their vegetative phase, they are thus able to move actively over a solid surface towards attractants or away from repellants . This essentially happens according to the principles of chemotaxis : the cells reverse their direction of movement at regular intervals. However, the time intervals change as soon as the bacteria are in a chemical gradient, i.e. in a concentration gradient of a substance that they can perceive. Viewed over a longer period of time, they move longer in the direction that is advantageous for them than in the opposite direction. It has been observed that Myxococcus cells leave a kind of "slime trail" of polysaccharides as they slide. Other cells of the Myxococcus swarm prefer to move along such mucus tracks, which favors group behavior.
If the cells move with the help of their pili , one speaks of S-motility . "S" stands for social . This form of movement occurs primarily when a swarm initiates its development program. Pili are protein threads that can be significantly longer than a cell itself. They are primarily used for attachment to a substrate, but in this case for attachment to other Myxococcus cells. This guarantees that the cells can maintain close contact, exchange biochemical signals and that all cells “know” what stage of development the swarm is in. The contact is so close that the connected cells are most likely fusing the outer membrane around them.
Myxobacteria represent a model for development and differentiation processes in the domain of bacteria. Several research laboratories around the world are working on the elucidation of the genetic and biochemical mechanisms that are necessary for the development process. In addition, it is expected that the genes for the production of numerous bactericidal substances will be found in the genome of the bacteria and it is hoped that these will be able to be used as antibiotic drugs. The way of life of the myxobacteria is also interesting because it is similar to that of some slime molds . Slime molds belong to the eukaryotes and not to the bacteria. An interesting example of convergent development .
Isolation and culture
Myxobacteria are widespread and relatively easy to isolate. For example, you can put small crumbs of soil or rabbit dung on a plate with water agar , which otherwise does not contain any nutrients, but to which a dense suspension of a bacterial culture (e.g. Escherichia coli ) has been applied beforehand . The activity of the myxobacteria becomes visible through the dissolving of the bacterial lawn in the vicinity of the soil pieces. After several days, the fruiting bodies emerge, with the help of which the myxobacteria can be isolated and assigned to specific genera. A pure culture is usually only possible in nutrient media that contain only casein or peptone and no carbohydrates. Sorangium cellulosum , a related genus, is an exception. This bacterium breaks down cellulose and can be cultivated on small pieces of filter paper.
