Bacteriology - Flagellar Movement

Flagellar Movement

                 Five major methods of movement have been observed: swimming movement conferred by flagella, flagella-mediated swarming, corkscrew movement of spirochetes, twitching motility associated with type IV pili, and gliding motility.
                                  Motile bacteria do not move aimlessly. Rather, motility is used to move toward nutrients such as sugars and amino acids and away from many harmful substances and bacterial waste
products.
                                   Movement toward chemical attractants and away from repellents is known as chemotaxis. Motile bacteria also can move in response to environmental cues such as temperature (thermotaxis), light (phototaxis), oxygen (aerotaxis), osmotic pressure (osmotaxis), and gravity.flagellar rotation results in two types of movement: a smooth swimming movement often called a run, which actually moves the cell from one spot to another, and a tumble, which serves to reorient the cell.

1. Swimming Movement by Flagella

  • The filament of a bacterial flagellum is in the shape of a rigid helix, and the cell moves when this helix rotates like a propeller on a boat.The flagellar motor can rotate very rapidly.
  • The E. coli motor rotates 270 revolutions per second (rps); Vibrio alginolyticus averages 1,100 rps.
  • To move forward in a run, the flagella rotate counterclockwise.
  • As they do so, the flagella bend at their hooks to form a rotating bundle that propels the cell forward. Clockwise rotation of the flagella disrupts the bundle and the cell tumbles.
  • EXAMPLE: For example, many bacteria with monotrichous, polar flagella use a counterclockwise
    rotation for a run (figure 3.44). When rotation is reversed, the cell tumbles. Many peritrichously flagellated bacteria operate in a somewhat similar way.

2. Swarming Movement

  • This motility occurs on moist surfaces and is a type of group behavior in which cells move in unison across the surface.
  • Most bacteria that swarm have peritrichous flagella.
  • Many also produce and secrete molecules that help them move across the substrate.

3. Spirochete Motility

  • In Spirochetes flagella do not extend outside the cell wall but rather remain in the periplasmic space and are covered by the outer membrane. They are called periplasmic flagella and are thought to rotate like the external flagella of other bacteria, causing the corkscrew-shaped outer membrane to rotate and move the cell through the surrounding liquid, even very viscous liquids
  • Flagellar rotation may also flex or bend the cell and account for the creeping or crawling movement observed when spirochetes are in contact with a solid surface.
  • EXAMPLE: Phylum Spirochaetes

4. Twitching and Gliding Motility

  • Twitching and gliding motility occur when cells are on a solid surface.
  • Gliding motility varies greatly in rate (from 2 to over 600 mm per minute) and in the nature of the motion.
  • Type IV pili are thought to alternately extend and retract to move cells during twitching motility.The extended pilus contacts the surface at a point some distance from the cell body.When the pilus retracts, the cell is pulled forward.Hydrolysis of ATP likely powers the extension/ retraction process.
  • In contrast to the jerky movement of twitching motility, gliding motility is smooth.
  • The gliding motility exhibited by Myxococcus xanthus is called adventurous (A) motility.
  • In Myxococcus xanthus, proteins similar to MotA and MotB of the flagellar motor function as the motors for gliding motility. They are thought to be in the plasma membrane (like flagellar motors)
    and are associated with other proteins, forming relatively large protein complexes. Additional proteins connect the motors to MreB, a cytoskeletal protein (section 3.6). Other proteins connect
    the motors to the substrate along which the cell glides. The motors are powered by PMF. How the action of the motor complexes are translated into gliding motility is still unknown.
  • EXAMPLE: Gliding  Motility - Flavobacterium spp. and Mycoplasma spp.Twitching Motility - Myxococcus xanthus

MOTOR:

  • The motor that drives flagellar rotation is located at the base of the flagellum.
  • Torque generated by the motor is transmitted to the hook and filament
  • The motor is composed of two components: the rotor and the stator. It is thought to function like an electrical motor.
  • The rotor turns in the center of a ring of electromagnets,the stator.
  • In typical Gram-negative bacteria, the rotor is composed of the MS ring and the C ring.
  • The C ring protein, FliG,The C ring protein, FliG, is particularly important because it is thought to interact with the stator.
  • The stator is composed of the proteins MotA and MotB, which form a channel through the plasma membrane. MotB also anchors MotA to cell wall peptidoglycan.
  • As with all motors, the flagellar motor must have a power source that allows it to generate torque and cause flagellar rotation.

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