Myxococcus xanthus: what is it?
M. xanthus is a soil bacterium which commonly grows in damp soil rich in organic matter. The bacteria are rod shaped and 10 times bigger than E. coli in size. They use peptides, lipids and other macromolecules for nutrition and tend to form large multicellular communities, which feed upon other microrganisms, utilizing extracellular antibiotics and degradative enzymes to immobilize and digest their prey (see movie in which Myxo (right) devours a colony of E. coli (left))(4, 5, 10).  
(M. xanthus cell and biofilm pictures
M. xanthus is one of many diverse Gram-negative bacteria which move by gliding motility. Gliding motility is traditionally described as movement in the direction of the long axis of the cell at a solid-liquid, solid-air or an air-liquid interface without the aid of flagella (7). M. xanthus has two genetically distinct systems for gliding. The first system is called social (S)-motility and involves the movement of cells in groups (3). The second system is called adventurous (A)-motility and involves the movement of single cells. S-motility requires type IV pili, lipopolysaccharide (LPS) O-antigen, and extracellular matrix polysaccharide (called fibrils). In particular, S-motility has been shown to be powered by type IV pili: pili are extruded from one cell pole and adhere to a surface or to another cell; retraction of the pilus then pulls the cell in the direction of the site of adhesion (1, 2, 6, 11). A- motility is not well understood. The traditional hypothesis assumes the extrusion of a polyelectrolyte gel to push the cell forward (12). A more recent model proposes that intracellular motor complexes, connected to both membrane-spanning adhesion complexes and to the cytoskeleton, power motility by pushing against the substratum and moving the cell body forward, much like focal adhesion-based traction or apicomplexan gliding motility in eukaryotic organisms (8).
M. xanhus cells periodically reverse their direction of gliding (see movie on the right) and cell reversal are thought to be required for directional adjustment as part of the biased random walk necessary for chemotaxis.
In nature, when M. xanthus swarming cells are unable to find sufficient nutrients, they enter a developmental process in which they aggregate forming raised pigmented mounds, termed fruiting bodies Within the fruiting bodies cells differentiate to form spores. Spores are metabolically dormant cells that can withstand prolonged periods of starvation, desiccation, and relatively high temperatures for their soil habitat. The process of sporulation involves the rounding up of the cells and the restructuring of the cell walls (5, 10). While the large majority of cells (80-90%) aggregate to form fruiting bodies, some cells follow a different developmental fate. They have been hypothesized to be scout cells, since spores are resting cells that cannot search for prey (9).
1.    Arnold, J. W., and L. J. Shimkets. 1988. Inhibition of cell-cell interactions in Myxococcus xanthus by congo red. J Bacteriol 170:5765-70.
2.    Bowden, M. G., and H. B. Kaplan. 1998. The Myxococcus xanthus lipopolysaccharide O-antigen is required for social motility and multicellular development. Mol Microbiol 30:275-84.
3.    Hodgkin, J., and D. Kaiser. 1977. Cell-to-cell stimulation of movement in nonmotile mutants of Myxococcus. Proc Natl Acad Sci U S A 74:2938-2942.
4.    Kaiser, D. 2006. A microbial genetic journey. Annu Rev Microbiol 60:1-25.
5.    Kaiser, D. 2003. Coupling cell movement to multicellular development in myxobacteria. Nat Rev Microbiol 1:45-54.
6.    Li, Y., H. Sun, X. Ma, A. Lu, R. Lux, D. Zusman, and W. Shi. 2003. Extracellular polysaccharides mediate pilus retraction during social motility of Myxococcus xanthus. Proc Natl Acad Sci U S A 100:5443-8.
7.    McBride, M. J. 2001. Bacterial gliding motility: multiple mechanisms for cell movement over surfaces. Annu Rev Microbiol 55:49-75.
8.    Mignot, T., J. W. Shaevitz, P. L. Hartzell, and D. R. Zusman. 2007. Evidence that focal adhesion complexes power bacterial gliding motility. Science 315:853-6.
9.    O'Connor, K. A., and D. R. Zusman. 1991. Behavior of peripheral rods and their role in the life cycle of Myxococcus xanthus. J Bacteriol 173:3342-55.
10.    Shimkets, L. J. 1999. Intercellular signaling during fruiting-body development of Myxococcus xanthus. Annu Rev Microbiol 53:525-49.
11.    Wall, D., and D. Kaiser. 1999. Type IV pili and cell motility. Mol Microbiol 32:1-10.
12.    Wolgemuth, C., E. Hoiczyk, D. Kaiser, and G. Oster. 2002. How myxobacteria glide. Curr Biol 12:369-77.