bacterial flagellum
/ bak-TEER-ee-ul fluh-JEL-um /
Many bacteria can swim, and to do it they grow a long, whip-like tail (or several) called a flagellum. Watch a swimming bacterium under a microscope and it looks like it is tumbling and darting through the water with purpose. The flagellum is the propeller that lets it do this — chasing food, fleeing poisons, and steering toward better conditions.
Here is the surprising part: the bacterial flagellum is not a flexible tail that wiggles like a sperm cell's. It is a stiff, corkscrew-shaped filament attached to an actual rotary motor embedded in the cell wall and membrane. The motor spins the filament around like a boat's propeller, driven by a flow of protons (positively charged hydrogen ions) across the membrane rather than by ATP directly. It can reverse direction, letting the cell switch between smooth swimming and random tumbling to change course. This makes it one of the very few true rotating wheels found anywhere in biology.
The flagellum matters both as a survival tool and as a famous case study in how complex molecular machines evolve. Its rotary motor is built from a couple of dozen different proteins assembled in a precise order. Important caution: the bacterial flagellum is completely different in structure from the flagella of eukaryotic cells (like the tail of a human sperm), which bend and beat using a totally separate machinery. Same name, unrelated machines — a classic source of confusion.
E. coli, the gut bacterium, sports several flagella that bundle together and spin counterclockwise to drive it smoothly forward. When the motor briefly reverses, the bundle flies apart and the cell tumbles in place, randomly picking a new direction — its way of 'deciding' which way to swim next.
E. coli alternates 'runs' and 'tumbles' to navigate, all driven by its rotary flagellar motor.
The bacterial flagellum is a rigid rotating propeller; the eukaryotic flagellum is a bending, beating structure. They share only the name, not the design.