Bacterial gliding is a process of motility whereby a bacterium can move under its own power. This process does not involve the use of flagella, pili, or fimbriae, all of which are more well-understood apparatuses used in bacterial motility. The exact mechanisms of gliding are currently unknown or only partially known. Generally, the process occurs whereby the cell, or organism, moves along a surface in the general direction of its long axis. Gliding may occur via distinctly different mechanisms, depending on the type of bacterium. This type of movement has been observed in phylogenetically diverse bacteria such as cyanobacteria, myxobacteria, cytophaga, flavobacteria, and mycoplasmas, and may play an important role in biofilm formation, bacterial virulence, and chemosensing.
Bacteria, as well as parasites, have the agility and means to evolve their motile operations in order to become accustomed to varying climates, water content, presence of other organisms, and firmness of surfaces or media. Gliding has been observed in a wide variety of phyla, and though the mechanisms may vary between bacteria, it is currently understood that it takes place in environments with common characteristics, such as firmness and low-water, which enables the bacterium to still have motility in its surroundings. Such environments with low-water content include biofilms, soil or soil crumbs in tilth, and microbial mats.
Gliding is distinct from other forms of motility occurring in semi-solid, moist, or aqueous environment; they do not appear to use mobile appendages such as pili or flagella attached to their cell body or cell surface. Such methods of motility with such appendages include swarming on softer semi-solid and solid surfaces (which usually involves movement of a bacterial population in a coordinated fashion via quorum sensing, using flagella to propel them), or twitching motility on solid surfaces (which involves extension and retraction of Type IV pili to drag the bacterium forward).
Gliding, as a form of motility, appears to allow for interactions between bacteria, pathogenesis, and increased social behaviours. Although the exact mechanism is debated upon in various studies, an example of this is observed in Myxococcus xanthus, a social bacterium which may employ A-motility (adventurous motility) as a proposed type of gliding motility, involving transient adhesion complexes fixed to the substrate while the organism moves forward. Other recent gliding mechanisms have proposed ejection or secretion of a polysaccharide slime from nozzles at either end of the cell body. Another suggests the presence of energized nano-machinery or large macromolecular assemblies located on the bacterium's cell body, while yet another utilize "focal adhesion complexes" and "treadmilling" of surface adhesins distributed along the cell body.
The gliding motility of Flavobacterium johnsoniae uses a helical track superficially similar to M. xanthus, but via a different mechanism. Here the adhesin SprB is propelled along the cell surface (spiraling from pole to pole), pulling the bacterium along 25 times faster than M. xanthus.
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