Myxococcus xanthus

Myxococcus xanthus is a gram-negative, rod-shaped species of myxobacteria that exhibits various forms of self-organizing behavior as a response to environmental cues. Under normal conditions with abundant food, it exists as a predatory, saprophytic single-species biofilm called a swarm. Under starvation conditions, it undergoes a multicellular development cycle. A swarm of M. xanthus is a distributed system, containing millions of entities 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. Likewise, 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 several hours. On the interior of the fruiting body, the rod-shaped cells differentiate 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. An important part of M. xanthus behavior is its ability to move on a solid surface by a mechanism called 'gliding'. Gliding Motility is a method of locomotion that allows for movement, without the help of flagella, on a solid surface. Gliding Motility is separated into two groups for the M. xanthus: A-motility (adventurous) and S-motility (social). In A motility, single cells move, resulting in a distribution with many single cells. In S motility, single cells do not move, but cells that are close to one another move. This leads to a spatial distribution of cells with lots of clusters and few isolated single cells. More than 37 genes are involved in the A-motility system, which comprises multiple motor elements that are arrayed along the entire cell body. Each motor element appears to be localized to the periplasmic space and is bound to the peptidoglycan layer. The motors are hypothesized to move on helical cytoskeletal filaments. Gliding force generated by these motors is coupled to adhesion sites that move freely in the outer membrane, and which provide a specific contact with the substratum, possibly aided by extracellular polysaccharide slime. 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. In response to starvation, myxobacteria develop species-specific multicellular fruiting bodies. 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 not as tolerant to environmental extremes as, say, 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. 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. 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 behaviour increasing its efficiency as a predator. This allows for a highly adaptive physiology that will have likely contributed to the near ubiquitous distribution of the myxobacteria