A flagellum, derived from the Latin word for “whip,” is a slender, complex appendage extending from the cell body of various microorganisms. Its primary biological function is to provide motility, allowing the organism to navigate its environment. This article focuses on the structure and function of the bacterial flagellum, a sophisticated nanoscale machine distinct from its eukaryotic counterpart. The bacterial flagellum operates as a propeller, enabling movement through liquid and semi-solid media, crucial for survival and colonization.
The Anatomy of Bacterial Flagella
The bacterial flagellum is a highly specialized structure composed of three distinct functional parts: the filament, the hook, and the basal body. The filament is the longest and most visible component, a rigid, helical structure extending out from the cell surface. It is constructed from thousands of subunits of a single protein called flagellin, which self-assembles into a hollow tube.
Connecting the filament to the cell envelope is the hook, a short, curved, and flexible joint. This hook acts as a universal joint, allowing the rotation generated inside the cell to be smoothly transferred to the rigid filament for propulsion. The entire apparatus is anchored into the bacterial cell wall and membrane by the basal body, which serves as the motor.
The basal body’s structure varies significantly between Gram-negative and Gram-positive bacteria. Gram-negative bacteria possess a more complex basal body, featuring four distinct protein rings that traverse the cell envelope: the L-ring (outer layer), the P-ring (peptidoglycan layer), and the inner MS-ring and C-ring. The MS-ring and C-ring are anchored to the plasma membrane and cytoplasm, respectively, forming the core of the rotary motor. Gram-positive bacteria, which lack an outer membrane, have a simpler basal body consisting only of the MS-ring and the C-ring.
Classification by Arrangement
The physical arrangement of flagella on the cell surface is a distinguishing characteristic used to classify motile bacteria.
A bacterium possessing a single flagellum located at one end of the cell is described as monotrichous. Lophotrichous bacteria feature a tuft or cluster of several flagella emerging from one pole. Amphitrichous bacteria have either a single flagellum or a tuft of flagella at both opposite poles of the cell. The final arrangement is peritrichous, characterized by numerous flagella distributed randomly over the entire cell surface. This widespread distribution requires a coordinated bundling of all flagella during movement to achieve a smooth “run.”
How Flagella Power Bacterial Movement
The bacterial flagellum functions as a true biological rotary engine. This rotary motor is powered not by ATP hydrolysis, but by the flow of ions across the cell membrane, specifically utilizing the Proton Motive Force (PMF). The PMF is the energy stored in the gradient of protons (hydrogen ions) concentrated outside the cell membrane.
The motor itself is composed of two main operational parts: the rotor (the C-ring and MS-ring of the basal body) and the stator units. The stator units, made of Mot proteins, are fixed in the membrane and act as the torque-generating elements. As protons flow through channels in the stator units, the energy released drives the rotation of the rotor and the attached flagellar filament at speeds that can exceed 100 revolutions per second.
This rotary mechanism dictates the two primary modes of bacterial movement, known as “run and tumble.” When all flagella rotate in a counter-clockwise (CCW) direction, their helical filaments coalesce into a single, cohesive bundle behind the cell. This coordinated rotation propels the bacterium in a relatively straight, smooth path, defined as a “run.”
The bacterium periodically interrupts this smooth movement by switching the flagellar motor’s rotation to the clockwise (CW) direction. This CW rotation causes the individual flagella within the bundle to fly apart, resulting in an erratic, random reorientation of the cell called a “tumble.”
Roles Beyond Simple Motility
While propulsion is the most apparent function, the bacterial flagellum serves several sophisticated roles intertwined with survival and pathogenicity. One important function is chemotaxis, a process where the bacterium directs its movement in response to chemical signals in the environment. This is achieved by regulating the frequency of the run and tumble phases based on environmental cues.
When a bacterium detects a favorable chemical attractant, such as a nutrient, it suppresses the motor’s switch to CW rotation, extending the duration of its smooth “run” in that direction. Conversely, moving away from an attractant or toward a repellent increases the frequency of “tumbles,” allowing the cell to rapidly reorient and search for a better path. This modulation of the run-tumble mechanism is controlled by a complex signaling cascade involving Che proteins, which ultimately influences the C-ring of the flagellar motor.
Flagella contribute significantly to the ability of pathogenic bacteria to cause disease. Motility allows pathogens like Helicobacter pylori to swim through the thick, viscous mucus layer of the stomach to colonize the underlying epithelial tissue. Flagella-driven movement is also essential for pathogens like Vibrio cholerae in navigating the intestinal tract and establishing infection.
The flagellum also plays a direct role in the initial stages of infection by facilitating adhesion to host cells and surfaces. The flagellin protein itself, or other associated flagellar proteins, can act as an adhesin, helping the bacterium stick to a suitable site for colonization. Furthermore, flagella are involved in the formation of biofilms by helping the bacteria find and initially attach to a surface.