Polar flagella are whip-like appendages that extend from the surface of various bacteria, serving as their primary means of movement. These specialized structures allow bacteria to navigate their surroundings, enabling them to seek out favorable conditions and avoid harmful ones. They act as propellers to generate thrust.
Structure and Arrangement
A polar flagellum is a complex assembly composed of three distinct parts: the filament, the hook, and the basal body. The filament extends helically outward from the cell surface and is made of flagellin proteins arranged in a hollow tube. This helical structure is responsible for generating propulsion when it rotates.
Connecting the filament to the bacterial cell is the hook, a curved, flexible segment that acts as a universal joint, allowing the filament to rotate freely. Anchoring the entire flagellar apparatus within the bacterial cell membrane and wall is the basal body. It consists of a rod surrounded by a series of protein rings, which vary in number depending on the bacterial type.
The term “polar” refers to the specific arrangement of these flagella, meaning they are located at one or both ends of the bacterial cell. Bacteria with a single flagellum at one pole are called monotrichous, while those with a tuft of flagella at one pole are lophotrichous. Some bacteria are amphitrichous, possessing a single flagellum or multiple flagella at both poles. This contrasts with peritrichous bacteria, which have flagella distributed all over their cell surface.
How Polar Flagella Propel Bacteria
The propulsion generated by polar flagella relies on a rotary motor embedded within the basal body. This molecular motor uses energy derived from an electrochemical gradient to rotate the flagellar filament. For instance, Vibrio species can rotate their flagella at speeds up to 100,000 revolutions per minute (rpm), propelling them through liquid at speeds up to 60 micrometers per second.
The rotation of the helical filament, much like a boat propeller, creates thrust that pushes the bacterium forward. Bacteria with a single polar flagellum exhibit a more linear, forward movement. However, bacteria with a tuft of flagella can alternate between pushing and pulling modes, and can even wrap their flagellar bundle around the cell body for slower movement.
This controlled movement allows bacteria to perform “taxis,” directed movement towards attractants or away from repellents. Chemoreceptors on the bacterial surface detect changes in chemical concentrations. These receptors then send signals to the flagellar motor, adjusting the direction and speed of flagellar rotation to guide the bacterium towards more favorable conditions or away from harmful ones.
Role in Bacterial Survival and Disease
Polar flagella confer advantages for bacterial survival in diverse environments and play a notable role in disease. Their ability to provide motility allows bacteria to actively seek out nutrient-rich areas, escape from predators or toxic substances, and colonize new habitats.
In the context of disease, polar flagella are recognized as virulence factors. For example, Vibrio cholerae utilizes its single polar flagellum to colonize the intestinal lining, which is a necessary step for establishing infection. Without a functional flagellum, its virulence is reduced.
Another example is Pseudomonas aeruginosa, which uses its polar flagellum to invade the respiratory tract and cause infections, particularly in individuals with cystic fibrosis. Flagella also contribute to the formation of biofilms, complex communities of bacteria that adhere to surfaces and are often more resistant to antibiotics and host immune defenses. By enabling initial surface attachment and movement within these structures, flagella indirectly support the persistence and spread of bacterial infections.