The bacterial flagellar motor is a complex, self-assembling rotary engine that provides propulsion for many species of bacteria. This sophisticated nanoscale machine allows bacteria to move rapidly and purposefully within fluid environments, a capability known as motility. The motor exhibits exceptional efficiency, sometimes approaching 100% in energy conversion, and spins its external filament propeller at speeds exceeding 100 revolutions per second. Its purpose is to convert electrochemical energy into mechanical rotation, enabling the organism to navigate its surroundings effectively.
The Core Architecture of the Bacterial Flagellar Motor
The flagellar motor is anchored within the bacterial cell envelope, a highly organized structure composed of over 30 different proteins. The entire assembly can be conceptually divided into the propeller, the universal joint, the driveshaft, and the embedded motor base. The outermost component is the filament, a long, helical structure made of flagellin protein subunits that acts as the propeller.
Connecting this filament to the internal motor is the hook, which functions as a flexible universal joint, transmitting torque while allowing for directional changes. A central rod acts as the driveshaft, passing through the cell membranes and connecting the hook to the motor’s rotating elements. The basal body, which houses the rotary machinery, consists of several concentric protein rings.
In Gram-negative bacteria, four rings support the structure. The L and P rings act as molecular bushings, supporting the rod as it passes through the outer membrane and the peptidoglycan layer, respectively. The MS (Membrane-Supramembrane) ring is embedded in the inner membrane and serves as a component of the rotor. The C (Cytoplasmic) ring is attached to the MS ring on the cell’s interior, completing the main rotating assembly, or rotor, and is involved in controlling the motor’s direction.
Generating Torque: The Proton Motive Force
The power source for this molecular machine is the electrochemical gradient across the inner membrane, often referred to as the Ion Motive Force (IMF). In many common bacteria, such as E. coli, this energy comes specifically from the Proton Motive Force (PMF), which is a gradient of protons (hydrogen ions). Other species, particularly marine bacteria like Vibrio, utilize a Sodium Motive Force, relying on a sodium ion gradient instead.
The motor’s rotation is generated by the stator units, which are protein complexes (such as MotA and MotB) positioned around the rotor. These stator units contain ion channels that open when they anchor to the peptidoglycan layer. The flow of ions down their concentration gradient and through these channels provides the energy that drives the rotation.
This ion flux causes conformational changes in the stator proteins, which then interact with the rotor’s C-ring, generating torque. This mechanism results in a true rotary motion. The speed of the motor is directly related to the strength of the ion motive force, with torque remaining constant up to high speeds.
The Sophistication of Directional Control
Bacterial movement is controlled by chemotaxis, which allows the organism to navigate toward chemical attractants and away from repellents. This navigation is achieved by rapidly switching the motor’s direction of rotation. The default movement is a “run,” where the flagellar motor rotates counter-clockwise (CCW).
During a run, the multiple flagellar filaments of peritrichous bacteria like E. coli coalesce into a single, cohesive bundle that propels the cell in a smooth, straight line. This movement continues as long as the bacterium senses it is moving toward a favorable environment. The motor can rapidly switch to clockwise (CW) rotation, a process that takes place in milliseconds.
Clockwise rotation causes the flagellar bundle to fly apart, resulting in a chaotic, erratic movement called a “tumble.” The tumble randomly reorients the cell, setting it up for a new run in a different direction. The frequency of these tumbles is controlled by a signaling cascade involving proteins like CheY. When phosphorylated, the CheY protein binds to the motor’s C-ring, inducing the conformational change necessary to promote CW rotation.
Relevance in Medicine and Nanotechnology
Understanding the flagellar motor holds significant implications for both public health and future engineering. Motility is an important factor in the pathogenicity of many bacteria, including Salmonella and pathogenic E. coli, as it enables them to spread and colonize host tissues. If scientists can disrupt the function or assembly of the motor, it could provide a new strategy for developing anti-virulence drugs that limit a pathogen’s ability to cause infection without directly killing it.
The flagellar motor serves as a model for nanotechnology due to its miniature size and exceptional efficiency. With a diameter of approximately 45 nanometers, this biological machine is a naturally occurring, self-assembling molecular device. Its ability to convert electrochemical energy into high-speed mechanical motion with near-perfect efficiency inspires the design of future nanoscale robotic devices, or “nanobots,” for applications in diagnostics, drug delivery, and microfluidics.