The bacterial flagellum is the most common and thoroughly studied prokaryotic motility structure. It resembles a spinning propeller-like structure that is used for swimming in aqueous environments and in some organisms enables swarming across solid surfaces. The flagellum is a very complex organelle consisting of over 20 proteins (flg, flh, fli, flj variants) and as many as 30 proteins assisting in regulation and assembly. Each Escherichia coli or Salmonella cell typically has 6-8 structures. The export system for assembly of the structure represents a classical Type III secretion system (T3SS). The main structure consists of 3 main substructures: the basal body, which anchors the structure in the cell membrane and contains the motor; the filament which acts as the propeller; and the hook, a joint which connects the basal body and filament. Rotation of the filament to generate movement is driven by the proton motive force, whereby H+ atoms crossing the cell membrane interact with the motor proteins (MotA, MotB), inducing a conformational change that turns the rotor. This rotation can reach speeds of 18,000rpm and propel the cell 25-35uM per second.
The rotation of the flagellum and the direction of movement is often regulated by sensory stimuli, allowing the cell to migrate towards attractive signals. In E.coli this is achieved through a signal transduction system that controls the phosphorylation state of the response regulator protein CheY. In the absence CheY-P the flagellum rotates CWW in a "run" state. The presence of CheY-P, signals induces a switch to CW rotation resulting in "tumbling". When an attractant binds to a receptor it initiates a conformational change and downstream cascade that leads to decreased levels of CheY-P, in response cells tumble less frequently and will move towards the attractant.
Some aquatic bacteria use hollow gas-filled vesicles to provide buoyancy and enable them regulate their position in the water column. Prototrophic bacteria may use the vesicles to find regions with appropriate light intensity, similarly aearobic bacteria may use them to float to oxygenated surface waters . Gas vesicles are commonly observed and studied in aquatic cyanobacteria, but have recently been discovered in Serratia sp., an enterobacterium . Typically 10-14 gvp genes are involved in vesicle formation. It has been shown that gas vesicles from Anabaena are permeable to H2, N2, 02, C02, CO, CH4, and Ar .
Many species of bacteria including Pseudomonas aeruginosa,Neisseria gonorrhoeae and Myxoccocus xanthus use a Type IV pilus (T4P) system for motility. Cell propulsion by T4P involves pilus extension, attachment, and then pilus retraction. This process results in a jerky pattern of movement which is why it has been named "Twitching motility".
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