-30 Some bacteria move by gliding motility
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- Gliding motility is an energy-requiring process that allows bacteria to move over a solid surface.
- Several different groups of microbes are capable of gliding motility including Myxococcus, Neisseria, Pseudomonas, Cytophaga, and Flavobacterium.
- There may actually be several mechanisms of gliding motility. One mechanism involves Type IV pili, using it as a tether. A second method involves extrusion of an extracellular polysaccharide. A final proposed mechanism may involve rotation of basal-body-like structure on the surface of the microbe.
Flagella are not the only means bacteria have developed for moving about the environment. A large collection of phylogenetically diverse bacteria have developed gliding motility, which is an energy-requiring process by which bacteria move smoothly over a solid surface. Gliding microbes tend to be predominately gram-negative, but there are examples of gram-positive bacteria. This type of motility is not as well understood as flagellar propulsion, and there appear to actually be several different mechanisms employed to accomplish it.
In one mechanism the microbes use pili (type iv pili specifically) that are extended away from the cell and stick to the surrounding surface. The microbe then pulls itself toward the tethered end by retracting the pilus back inside the cell. By repeating this process the cell drags itself along a surface. This type of motility is observed in a number of microorganisms including Pseudomonas aeruginosa, Neisseria gonorrhoeae, and Myxococcus xanthus. Movement in this manner requires energy in the form of ATP.
There are also gliding bacteria that use other mechanisms. Some filamentous cyanobacteria seem to extrude an extracellular polysaccharide through small pores on the surface of the organism. Figure 2-48 depicts this type of gliding motility. It is thought that the polysaccharide exiting from the cell propels the microbe along the surface by some means.
Figure 2.48. Gliding motility in Paenibacillus. Colonies of vegetative cells of a Paenibacillus isolate moving across an agar surface using gliding motility. (Source: Jon Roll, University of Wisconsin-Madison)
A third groups of microbes (Cytophaga and Flavobacterium) seem to use yet another mechanism that is dependent upon proton motive force. Observation of these microbes in the presence of very small latex beads show the beads moving along the surface of the microbe in a directional manner. The bead moves from one pole of the rod-shaped bacteria to the other, sometimes reversing direction. One model to explain this behavior is the concerted movement of cytoplasmic membrane proteins that are coupled to outer membrane proteins. These membrane proteins might form a kind of conveyor belt. The outer membrane proteins in this mechanism are in contact with the environmental surface and their movement propels the microbe forward.
Interestingly, Myxococcus xanthus uses several of these mechanisms in its motility. M. xanthus is a social predator. It glides around in large groups of cells secreting toxins and degradative enzymes that kill other microbes. The leftovers of these dead microbes then provide nutrients for the marauding Myxococcus. Investigations by Dale Kaiser and others have revealed that this microbe actually has two types of gliding motility, social and adventurous. Social motility occurs in groups while adventurous motility involves single cells. It turns out that social motility is dependent upon pili as described above, while adventurous motility involves the extrusion of slime from pores in the microbe's cell wall. If you want to see a close relative of these microbes in action, there is a [Prev]