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Gliding motility is a type of translocation used by microorganisms that is independent of cell-surface appendages such as flagella, pili, and fimbriae. [1] Gliding allows microorganisms to travel along the surface of thin aqueous films. The mechanisms of this motility are only partially known.
Bacterial gliding allows motility along a thinly aqueous surface. The other very different type of bacterial motility is called twitching motility that also allows movement along a thinly aqueous surface, but this type of movement is jerky and uses type IV pili as its means of transport.
The speed of gliding varies between organisms, and the reversal of direction is seemingly regulated by some sort of internal clock. [2] For example the apicomplexans are able to travel at fast rates between 1–10 μm/s. In contrast Myxococcus xanthus bacteria glide at a rate of 0.08 μm/s. [3] [4]
Bacterial gliding is a process of motility whereby a bacterium can move under its own power. Generally, the process occurs whereby the bacterium moves along a surface in the general direction of its long axis. [5] Gliding may occur via distinctly different mechanisms, depending on the type of bacterium. This type of movement has been observed in phylogenetically diverse bacteria [6] such as cyanobacteria, myxobacteria, cytophaga, flavobacteria, and mycoplasma. The first report of gliding motility in the Desulfobulbaceae family has been shown in cable bacteria. [7]
Bacteria move in response 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. [5]
Bacterial gliding may be achieved by:
a) Type IV pili: A cell attaches its type IV pili to a surface or object in the direction it is traveling. The proteins in the pili are then broken down to shrink the pili pulling the cell closer to the surface or object that it was attached to. [8]
b) Specific motility membrane proteins: Transmembrane proteins are attached to the host surface. This adhesion complex can either be specific to a certain type of surface like a certain cell type or generic for any solid surface. Motor proteins attached to an inner membrane force the movement of the internal cell structures in relation to the transmembrane proteins creating net movement. [9] This is driven by the proton motive force. [10] The proteins involved differ between species. An example of a bacterium that uses this mechanism would be Flavobacterium. This mechanism is still being studied and is not well understood. [11]
c) Polysaccharide jet: The cell releases a 'jet' of polysaccharide material behind it propelling it forward. This polysaccharide material is left behind. [12] Cell-invasion and gliding motility have TRAP (thrombospondin-related anonymous protein), a surface protein, as a common molecular basis that is both essential for infection and locomotion of the invasive apicomplexan parasite. [13] Micronemes are secretory organelles on the apical surface of the apicomplexans used for gliding motility.
The mechanism of gliding might differ between species. Examples of such mechanisms include:
Swarming motility occurs 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 [6] on solid surfaces (which involves extension and retraction of type IV pili to drag the bacterium forward). [21]
Gliding, as a form of motility, appears to allow for interactions between bacteria, pathogenesis, and increased social behaviours. It may play an important role in biofilm formation, bacterial virulence, and chemosensing. [15]
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