The bacterial flagellum is a highly sophisticated, self-assembling biological machine that provides many bacteria with the power of movement. This complex, whip-like appendage functions as a microscopic propeller that extends from the bacterial cell surface. Its primary function is to propel the bacterium through liquid environments, enabling it to seek out nutrient-rich areas or escape harmful conditions. Motility is a fundamental requirement for the survival and propagation of countless bacterial species. This structure represents a remarkable feat of nanotechnology, converting chemical energy into mechanical rotation with speed and precision.
Anatomy and Assembly
The bacterial flagellum consists of three main components: the long, helical filament, the flexible hook, and the basal body which anchors the entire apparatus to the cell envelope. The filament acts as the main propeller; it is a rigid, hollow tube made of thousands of flagellin protein subunits arranged helically. The hook is a short, curved junction that functions much like a universal joint, transmitting the motor’s torque to the propeller.
The basal body is the most complex component, embedded within the cell membrane and cell wall, serving as both the anchor and the rotary motor. Building this machine requires a highly coordinated, multi-step process that occurs from the inside of the cell outward. Assembly begins with the formation of the innermost rings, such as the MS-ring, which is incorporated into the cytoplasmic membrane. Subsequent components are synthesized in the cytoplasm and transported sequentially through a central channel.
A specialized Type III secretion system pushes these protein subunits through the narrow channel to the distal end where they self-assemble. The rod is built first, followed by the hook, and finally, the flagellin subunits form the long external filament. This “inside-out” construction mechanism ensures that the motor’s anchor is fully secured before the external propeller structure is completed.
The Rotary Motor Mechanism
The basal body functions as a true rotary motor, spinning the external filament rapidly, rather than using a back-and-forth whipping motion. This motor is powered by a transmembrane electrochemical gradient known as the ion motive force, which provides the energy for rotation. In most well-studied bacteria, such as Escherichia coli, this force is generated by the flow of protons (hydrogen ions) across the cell membrane, called the proton motive force (PMF).
The motor consists of a fixed stator and a rotating rotor complex. Stator units, composed of proteins like MotA and MotB, function as ion channels anchored to the peptidoglycan layer of the cell wall. As protons flow through the stator channels, the energy released is coupled to the rotation of the rotor complex. In certain species, like the marine bacterium Vibrio, a sodium ion gradient is used instead of the PMF.
The speed of this rotary motor is remarkable, with some flagella rotating at hundreds of revolutions per second. This rapid rotation allows a bacterium to travel up to 60 cell lengths per second in a fluid environment. Torque is generated by the physical interaction between the flowing ions and the rotor components, specifically the C-ring, which drives the entire propeller assembly.
Directed Motion and Chemotaxis
Bacterial movement is directed by a process called chemotaxis, which allows the cell to sense and respond to chemical gradients in its environment. The movement pattern is characterized by alternating phases known as “runs” and “tumbles.” A “run” occurs when the flagellar motor rotates counter-clockwise (CCW), causing the multiple flagella to coalesce into a single bundle that pushes the bacterium forward in a straight line.
When the motor switches to clockwise (CW) rotation, the flagellar bundle separates. The bacterium stops moving forward and undergoes a chaotic reorientation known as a “tumble,” which resets the cell’s direction. Chemoreceptors on the surface constantly monitor the concentration of chemicals, such as nutrients (attractants) or toxins (repellents).
If a bacterium senses it is moving toward a higher concentration of an attractant, its internal signaling pathway suppresses tumbling. This results in the prolongation of the “run” phase, biasing movement in the correct direction. Conversely, if conditions worsen, a signaling protein called CheY-P accumulates, interacting with the motor’s switch complex to induce CW rotation and a more frequent “tumble.”
Role in Bacterial Virulence
The flagellum is a significant factor contributing to a bacterium’s ability to cause disease, or its virulence. The ability to move toward a host cell or a specific tissue site is often the first step in establishing an infection. For instance, pathogens like Helicobacter pylori use their flagella to drill through the viscous mucus layer of the stomach to reach the underlying epithelial cells.
The flagellum also facilitates adhesion, allowing the bacterium to anchor itself to host tissues, acting like a microscopic grappling hook. Flagellin, the filament’s protein subunit, can directly bind to host cell surfaces, promoting initial colonization and the subsequent formation of biofilms. Flagellar motility further aids in the invasion process, helping certain bacteria penetrate deeper into host tissues after initial attachment.
However, the flagellum can also be a liability because flagellin is a potent trigger for the host immune system. The innate immune system possesses specialized receptors, such as Toll-like receptor 5 (TLR-5), that recognize flagellin, initiating an inflammatory response to eliminate the invading pathogen. This immune recognition has driven some successful pathogens to regulate or modify their flagella to evade detection while still maintaining the benefits of motility and adhesion.