Flagellar propulsion, mechanism

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Bacterial flagella are made of flagellin; its mechanism is rotation in which a rotor at the base of the flagella drives the rotation, powered by a proton or chemical gradient.

Flagella are structures used by cells to move in aqueous environments. Bacterial flagella act like propellers. They are stiff spiral filaments composed of flagellin protein subunits that extend outward from the cell and spin in solution. The basal body is the motor for the flagellum and embeds in the plasma membrane. A hook region connects the basal body to the filament. 

A diagram showing the attachment point of flagella in gram-positive and gram-negative bacteria. The gram-positive diagram shows the filament on the outside of the cell wall; a bent elbow labeled hook connects the filament to the cell wall. A thin line between the filament and hook is labeled junction. The hook connects to a rod which connects to a basal body in the inner membrane. The basal body is a complex structure with a C-ring on the bottom. In the center of this ring is a sphere labeled type III secretion protein. Outside of this are oval structures labeled stator. On top of the secretion protein is a ring labeled MS-ring. The gram-negative flagellum is similar. There is a filament attached to a junction attached to hook. In the outer membrane is a ring labeled L-ring that connects to a rod in the periplasmic space. A P-ring sits in the cell wall. In the inner membrane is the C-ring, type III secretion system, MS ring and stator.

Bacterial Flagellum. The basic structure of a bacterial flagellum consists of a basal body, hook, and filament. 

Directional movement depends on the arrangement of the flagella. Bacteria can move in response to a variety of environmental signals, including light and chemical gradients, such as sodium. A movement toward a chemical attractant or away from a repellent is achieved by increasing the length of runs and decreasing the length of tumbles.

When running, flagella rotate in a counterclockwise direction, allowing the bacterial cell to move forward. When tumbling, flagella spread out while rotating in a clockwise direction, creating a looping motion and preventing meaningful forward movement but reorienting the cell.

A diagram showing the run and tumble of bacterial motion. The tumble is when a clockwise rotation of flagella cause the bacterial cell to tumble about. The run is when a counter-clockwise rotation of the flagella cause the bacterial cell to move in a linear direction.

Rotation of Flagella. 

When no chemical gradient exists, the lengths of runs and tumbles are equal, and the overall movement is more random.

A diagram showing the run and tumble motion of bacteria. In the run, the bundeled flagella move in a counter clockwise rotation and the cell moves in a straight line. In the tumble, the flagella separate due to a clockwise rotation and the bacterial cell floats with no particular direction. This is followed by another run. If there is an attractant (a chemical gradient) the bacterial cell moves towards the attractant and the length of the run is extended.

Chemical Gradient. Without a chemical gradient, flagellar rotation cycles between counterclockwise (run) and clockwise (tumble) with no overall directional movement. However, when a chemical gradient of an attractant exists, the length of runs extends, while the length of tumbles decreases.


Key Points

• Some prokaryotic cells use one or more flagella to move through the water. Peritrichous bacteria, which have numerous flagella, use runs and tumbles to move purposefully in the direction of a chemical attractant.

• As the flagella rotate in a counterclockwise direction, it bundles; however, it tumbles when turning in a clockwise direction.

• The bacterial flagellum is driven by a rotary engine made up of protein powered by a proton motive force.


Key Terms

flagellum: a hairlike structure that acts primarily as an organelle of locomotion

flagellin: a globular protein that arranges itself in a hollow cylinder to form the filament in a bacterial flagellum

basal body: is a protein structure found at the base of a flagellum

run: flagella rotate in a counterclockwise direction allowing the bacterial cell to move forward

tumbling: flagella spread out while turning in a clockwise direction preventing movement 

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