Bacteria swim with a rotary motor that can spin 100,000 rpm
A bacterial flagellum is a genuine rotating machine: a protein motor, driven by a flow of protons across the membrane, turns a helical propeller. Unloaded, the rotor can reach 6,000 to 100,000 revolutions per minute, and it can reverse direction almost instantly. Its gears even mesh with a measured ratio of about 6.2.
Flagellum is Latin for whip, after the lashing motion. Bacteria, archaea and eukaryotes all have flagella for getting around, but the three kinds differ in structure, proteins and propulsion. Bacterial and archaeal versions rotate; the archaeal one is called an archaellum and is not considered evolutionarily related despite its rotary motor. Eukaryotic flagella, found on sperm and many protists, are membrane-wrapped extensions built on microtubules and the motor protein dynein, and they bend back and forth. Motile cilia share their structure but are shorter.
Building a bacterial flagellum takes more than 50 coordinated proteins. The filament is a hollow helical tube of flagellin about 20 nanometres thick, usually made of 11 strands lying roughly along its axis, though Campylobacter jejuni has seven. A sharp bend called the hook points the helix away from the cell, and a shaft passes through protein rings in the envelope that work like bearings: two rings in gram-positive bacteria, four in gram-negative ones.
The motor sits at the inner membrane. Between 11 and 16 stator units, each a small proton-powered motor, engage the rotor like interlocking gearwheels, and more are recruited when greater torque is needed. With the filament attached, speeds typically drop to 200 to 1,000 rpm. Some Vibrio species use sodium ions rather than protons. A slight shift in a rotor protein called FliG flips the direction, and because there is no off switch, a protein called EpsE can act as a clutch, letting a bacterium stay put.
Flagella can serve other purposes: Salmonella typhimurium uses them to sense wetness. They also matter for disease; Helicobacter pylori swims across the stomach to its protective mucus layer, and settling there can lead to gastritis and ulcers. Similarities between the flagellar base and the type three secretion system, a molecular injection device, support the idea that flagella evolved from it.
Source: Flagellum
