What Is a Polar Flagellum and How Does It Work?
Explore how a bacterium's polar flagellum works as a complex rotary engine, enabling controlled movement and navigation in response to its environment.
A polar flagellum is a specialized appendage on certain bacteria that acts much like a propeller on a boat. This structure extends from the cell surface and is involved in its ability to move through liquid environments. The primary purpose of this motility is to navigate toward beneficial conditions and avoid harmful ones. This movement is a direct result of the flagellum’s structure and rotational mechanism.
Anatomy of the Flagellar Motor
The bacterial polar flagellum is composed of three main parts: the basal body, the hook, and the filament. The basal body is the engine of the flagellum, embedded within the cell’s membranes and wall. This component functions as a rotary motor, harnessing cellular energy to generate the torque for rotation. It consists of a series of protein rings that act as bearings, allowing a central rod to spin.
Connecting the internal motor to the external filament is the hook, a short, curved, and flexible structure. The hook acts as a universal joint, transmitting rotation from the basal body to the filament. Its flexibility allows the long filament to extend away from the cell surface at an angle, which is necessary for efficient propulsion.
The most prominent part of the flagellum is the long, helical filament. This structure is composed of thousands of copies of a protein called flagellin, which assemble into a hollow, corkscrew-like tube. The filament’s rigid and helical shape allows it to function like a propeller. As it rotates, it displaces the surrounding fluid, creating thrust that moves the bacterium.
Arrangements of Polar Flagella
Bacteria exhibit several distinct arrangements of polar flagella that are specific to different species. The simplest arrangement is monotrichous, where a single flagellum is located at one pole of the bacterial cell. A classic example is Vibrio cholerae, the organism responsible for cholera. This solitary flagellum provides straightforward, directional motility.
Another arrangement is lophotrichous, characterized by a tuft of multiple flagella at a single pole. These flagella rotate together as a coordinated bundle to propel the bacterium. This bundling action generates a more powerful thrust than a single flagellum could achieve. Pseudomonas fluorescens is a species with this flagellar arrangement.
A third variation is the amphitrichous arrangement, where the bacterium has a flagellum at both poles. These flagella can rotate independently or in coordination to control the cell’s movement. The bacterium Spirillum volutans possesses this system, allowing for rapid reversals in direction and a high degree of maneuverability.
Mechanism of Locomotion
The movement generated by a polar flagellum is not a whip-like lash, but a rotation. The entire structure, from the basal body to the tip of the filament, spins like a corkscrew. This rotation is powered by a biological engine. The energy source is not ATP, but a flow of ions across the bacterial cell membrane.
This power source is the proton motive force. Protons are pumped out of the bacterial cytoplasm, creating a higher concentration of protons outside the cell than inside. This gradient creates an electrochemical potential. As protons flow back into the cell through a channel in the basal body, they drive the rotation of the motor’s rings.
The direction of rotation, either clockwise or counter-clockwise, determines the direction of bacterial movement. For a bacterium with a single polar flagellum, counter-clockwise rotation propels the cell forward in a smooth, straight line called a “run.” Reversing the rotation to clockwise can cause the bacterium to pull backward or reorient itself, allowing it to change direction. Some flagellar motors can achieve speeds of up to 100 rotations per second.
Role in Bacterial Behavior
The ability to move is not random; it is a regulated behavior that allows bacteria to survive. The primary driver for this directed movement is a process known as chemotaxis. This is the mechanism by which bacteria sense chemical gradients and move toward beneficial substances, like nutrients, and away from harmful ones, like toxins. The flagellum is the direct instrument of this response.
Cellular receptors on the bacterial surface detect specific chemical attractants or repellents. This information is processed through an internal signaling pathway that controls the direction of the flagellar motor’s rotation. If a bacterium senses it is moving toward a food source, the pathway promotes counter-clockwise rotation, resulting in longer runs in that direction.
Conversely, if the bacterium detects a repellent or a decreasing attractant, the signaling system triggers a change in the motor’s direction. This involves a switch to clockwise rotation, which causes the bacterium to tumble or change its orientation. Following this reorientation, the bacterium begins a new run in a random direction. This biased random walk allows the bacterium to navigate toward more favorable environments.