1. Introduction
The bacterial flagellum is one of the most efficient and most studied propulsion systems in biology. Its helical shape and rotation are essential to its function: it is a rotating helix, not a beating whip. This mechanism is fundamentally different from eukaryotic cilia and flagella.
2. Structure of the Bacterial Flagellum
The bacterial flagellum is composed of three parts: the basal body (anchored in the membrane), the hook (flexible junction structure) and the helical filament (the propulsive part). The filament is a polymer of flagellin, a protein that self-assembles into a hollow helix about 20 nm in diameter. Its helical shape is determined by the flagellin structure and can change depending on conditions.
3. The Rotary Molecular Motor
The basal body contains a rotary motor powered by the proton gradient (or sodium in some species) across the membrane. This motor can rotate at 100-1000 revolutions per second in both directions. When flagella rotate counterclockwise (viewed from behind), they bundle together and propel the bacterium in a straight line. When some flagella reverse rotation, the bundle disassembles and the bacterium tumbles randomly.
4. Swimming and Chemotaxis
Bacterial swimming alternates between "run" phases (straight swimming) and "tumble" phases (random reorientation). Chemotaxis — the ability to swim toward attractants and away from repellents — is achieved by modulating tumble frequency: the bacterium tumbles less often when moving toward an attractant, producing net movement in the right direction.
5. Conclusion
The bacterial flagellum is a remarkable example of evolved molecular engineering: a nanometric rotating helix powered by an electrochemical gradient, capable of efficient propulsion in a viscous medium at low Reynolds number. Its helical shape is functionally essential, not ornamental.