Bacteria are microscopic, single-celled organisms found in diverse environments. Many move independently, often using a sophisticated biological machine known as the bacterial flagellar motor. This motor represents a remarkable feat of nanoscale engineering, showcasing complexity and efficiency in its design and operation. It allows bacteria to navigate their surroundings, making it a fundamental mechanism for locomotion. The flagellar motor is a self-assembling complex, typically around 45 nanometers in diameter, constructed from approximately 20 different proteins.
The Motor’s Architecture
The bacterial flagellar motor is embedded within the bacterial cell envelope and consists of several distinct structural components. At its core is the rotor, which includes the MS-ring and the C-ring, acting as the base for the motor’s assembly. The MS-ring (FliF protein) is a transmembrane complex, while the C-ring (composed of FliG, FliM, and FliN proteins) attaches to the MS-ring’s cytoplasmic face and is involved in controlling rotation.
The stator units, which are the force generators, surround the rotor and are embedded in the cytoplasmic membrane. In bacteria like E. coli and Salmonella, each stator unit is typically formed by two MotB subunits and four or five MotA subunits. These stator units act as stationary components, providing the necessary anchoring and force generation for rotation. The MotB subunit contains a domain that binds to the peptidoglycan layer, anchoring the stator to the cell wall.
Extending from the rotor is the rod, which serves as a central drive shaft, connecting the MS-ring to the hook. The L-ring and P-ring are also part of this axial structure, acting as bushings or bearings in the outer membrane and peptidoglycan layer, respectively, to allow the rod to rotate smoothly. Finally, the hook, a flexible universal joint, connects the rod to the external helical filament. The long, helical filament, which can extend several micrometers beyond the cell surface, functions as the propeller.
Powering Bacterial Movement
The bacterial flagellar motor generates rotational force by harnessing the flow of ions across the cell membrane. This process relies on a transmembrane electrochemical gradient of ions, primarily protons (H+) or, in some species like Vibrio, sodium ions (Na+), known as the proton-motive force (PMF) or sodium-motive force (SMF). The stator units, composed of MotA and MotB proteins, function as ion channels.
As ions flow through the MotB channels, they interact with the stator and rotor components. A conserved aspartic acid residue (Asp32 in E. coli) within the transmembrane segment of MotB is thought to be the proton-binding site. The binding and dissociation of these ions are believed to trigger conformational changes in the cytoplasmic domain of MotA, which then interact with the FliG protein on the C-ring of the rotor. These interactions generate power strokes, causing the rotor to spin.
The rotation of the rotor is then transmitted through the rod and the flexible hook to the external helical filament, causing the filament to rotate like a propeller. The flagellar motor can rotate at remarkable speeds, reaching up to 50,000 RPM in some cases. The direction of flagellar rotation is precisely controlled by the chemosensory system, which enables bacteria to switch between clockwise (CW) and counter-clockwise (CCW) rotation almost instantaneously.
In bacteria like E. coli and Salmonella, counter-clockwise rotation causes the flagella to form a bundled propeller, propelling the bacterium forward in a “run”. When the flagella rotate clockwise, the bundle disassociates, causing the bacterium to “tumble” and randomly reorient itself. This ability to switch between runs and tumbles allows bacteria to change direction and navigate their environment effectively.
Why the Flagellar Motor is Important
The bacterial flagellar motor plays a profound role in bacterial survival and adaptation within diverse environments. This motor allows bacteria to exhibit chemotaxis, the ability to move towards beneficial attractants, such as nutrients, and away from harmful repellents or toxins. This directed movement is a sophisticated strategy for efficient nutrient acquisition and locating optimal niches for growth, especially in carbon-poor conditions.
Beyond environmental navigation, flagellar motility is often a significant factor in bacterial virulence and pathogenicity. For many bacterial pathogens, the ability to move is necessary for them to effectively infect host organisms and colonize tissues. For instance, motility can be particularly important for bacteria residing in the gastrointestinal tract or for those needing to invade host cells. Disrupting flagellar function can reduce the ability of certain bacteria to cause disease.
Understanding the intricate mechanics of this nanoscale motor also has broader implications for scientific research and technological innovation. The bacterial flagellar motor serves as a powerful model system for studying molecular machines and inspires fields like nanotechnology, for designing and building artificial nanomachines. Insights into the flagellar motor’s structure and function are contributing to the development of new antimicrobial strategies. By targeting the flagellar motor or the chemotaxis signaling pathway, scientists are exploring novel ways to combat bacterial infections, by interfering with bacterial movement and colonization, rather than directly killing them.