Microbiology
The Bacterial Flagellar Motor: Nature's Rotary Engine
Bolt a fishing line to a doorknob-sized turbine, spin it at 100,000 revolutions per minute, and let it reverse direction in less than a millisecond — then shrink the whole assembly until it is 45 nanometers wide, small enough that 200 of them could line up across a single human red blood cell. That is the bacterial flagellar motor: a reversible, self-assembling, proton-powered rotary engine built from about 30 proteins, embedded in the cell envelope of Escherichia coli, Salmonella, and thousands of other bacteria.
It is the only true wheel-and-axle rotary machine that biology evolved, predating human invention by billions of years. Each motor drives a helical propeller — the flagellum — that can be tens of micrometers long, pushing a cell forward at roughly 25 body-lengths per second while burning nothing but the electrochemical gradient of protons across the membrane.
- Motor diameter≈ 45 nm
- Proteins~30 distinct types
- Rotation (E. coli)~100–300 Hz
- Max speed (Vibrio, Na⁺)up to ~1,700 Hz
- Peak torque~1,300–1,400 pN·nm
- Switch time< 1 ms (CW⇄CCW)
Interactive visualization
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A condensed visual walkthrough — narrated, captioned, under a minute.
A rotary engine, not a whipping tail
For decades the flagellum was pictured as a flexible tail that bacteria lashed like a swimmer's kick. The truth, established in 1973–1974 by Howard Berg and by Michael Silverman and Melvin Simon, is stranger and more elegant: the filament is a rigid corkscrew, and the cell rotates it about its base like a ship's propeller. In the decisive tethering experiment, a filament was glued by an antibody to a glass slide so the immobilized flagellum forced the entire cell body to spin — visibly, under the microscope — proving that the machine at the base is a genuine rotary motor, complete with a rotor and stator.
The whole engine spans the Gram-negative cell envelope. From the inside out it comprises the MS-ring and C-ring in and just below the inner membrane, a rod acting as a driveshaft, the L-ring and P-ring that serve as bushings through the outer membrane and peptidoglycan, a flexible hook that works as a universal joint, and finally the long helical filament — the propeller itself.
The parts list: rotor, stator, driveshaft, propeller
The motor is a modular assembly of about 30 protein species. The load-bearing structures fall into a few functional groups:
- Rotor (the spinning part). The MS-ring is a 34-fold ring of the protein FliF in the inner membrane. Beneath it hangs the C-ring (cytoplasmic ring), built from FliG, FliM, and FliN. FliG carries the charged residues that physically engage the stator and generate torque; FliM and FliN form the switch that decides which way the motor turns.
- Stator (the fixed part that pushes). Each torque-generating unit is a MotA₅MotB₂ complex — five copies of MotA wrapped around two copies of MotB — anchored to the rigid peptidoglycan cell wall through MotB's periplasmic domain. Up to about 11 stators can dock around one motor.
- Driveshaft and bushings. The rod (FlgB, FlgC, FlgF, FlgG) transmits rotation outward; the P-ring (FlgI) and L-ring (FlgH) form bearings through the cell wall and outer membrane.
- Hook and filament. The hook (~120 copies of FlgE) is a curved universal joint ~55 nm long; the filament is ~20,000 copies of a single protein, flagellin (FliC), polymerized into a hollow helical tube ~20 nm wide that can reach 10–15 µm in length.
How torque is generated, ion by ion
The motor is powered not by ATP but by the proton-motive force (PMF) — the same electrochemical gradient that drives ATP synthase and the whole of chemiosmosis. In E. coli the PMF is roughly −150 to −170 mV; protons want to flow inward, and the motor turns that flux into rotation.
Each MotA₅MotB₂ stator forms two proton half-channels. The critical residue is a conserved aspartate, Asp32 of MotB: a proton binding to it triggers a conformational change that ratchets the MotA pentamer around by one step. Recent cryo-EM structures (2020, from the labs of Kojima, Blair, and others) revealed the beautiful surprise — the pentameric MotA ring itself spins around the central MotB dimer, and its inner surface bears charged residues that grip the FliG ring like a gear engaging a gear.
- Ion binds Asp32 → MotA pentamer advances one step → its charged loop pushes on a complementary charge on FliG → the rotor turns.
- Roughly 37 protons pass per revolution, and each of ~34 FliG subunits is engaged in turn, so a full turn is the sum of thousands of tiny electrostatic shoves.
- Multiple stators act in parallel and independently, so torque scales roughly linearly with stator number — a motor with 11 stators produces about 11× the push of a single unit.
Speed, torque, and a self-tuning gearbox
The numbers are staggering for something built from soft matter. A proton-driven E. coli motor spins at ~100–300 Hz (6,000–18,000 rpm), while sodium-driven motors in Vibrio alginolyticus reach ~1,700 Hz — over 100,000 rpm, faster than a jet engine's turbine. Efficiency at low speed approaches an extraordinary 90–100% of the thermodynamic limit.
The torque–speed relationship is a near-flat plateau at low speed (~1,300 pN·nm in E. coli) that falls off sharply near the maximum. This lets the motor deliver maximum force exactly when the load is heaviest — for example when the cell is stuck in viscous medium. Even more remarkably, the motor is a mechanosensor with a self-adjusting gearbox: when the external load rises, stator units are recruited from a membrane pool and dock onto the motor within seconds; when load drops, they diffuse away. The stator complement is dynamically tuned, on the fly, to match the job.
Reversing the engine: chemotaxis and run-and-tumble
The motor's second astonishing trick is instantaneous reversal. E. coli swims by bundling several counterclockwise-rotating flagella into a single helical propeller — a smooth run. When one or more motors flip to clockwise, the bundle flies apart and the cell tumbles, randomly reorienting. Alternating runs (~1 s) and tumbles (~0.1 s) produces a biased random walk that carries the cell up gradients of nutrients — chemotaxis.
The switch is throw by the phosphorylated response regulator CheY-P. Membrane chemoreceptors control the kinase CheA, which phosphorylates CheY. CheY-P diffuses to the motor and binds FliM in the C-ring; enough bound molecules cooperatively snap the entire ring into its clockwise conformation, reversing which face of FliG the stators push against. The switch is ultrasensitive (Hill coefficient ~10) and completes in under a millisecond. An adaptation system — methylation of the receptors by CheR and CheB — resets sensitivity so the cell responds to changes in concentration rather than absolute levels, giving it a chemical memory a few seconds long.
Self-assembly through its own hollow core
The flagellum builds itself from the inside out through a dedicated type III secretion system (T3SS) at its base — the same molecular machinery that pathogenic bacteria repurpose to inject toxins into host cells. Flagellin subunits are never released into the medium; they are threaded, unfolded, down the 2-nm central channel of the growing filament and add to the distal tip under a cap protein (FliD) that chaperones their folding.
- Because subunits must diffuse the entire length of the filament, growth slows as the filament lengthens — a filament tens of micrometers long can take an hour to complete.
- Assembly is strictly ordered by a molecular ruler: the protein FliK measures hook length (~55 nm) and triggers a secretion-specificity switch that stops exporting hook subunits and starts exporting flagellin.
- Gene expression is tiered into early, middle, and late classes governed by the master regulator FlhDC and the flagellar sigma factor FliA (σ²⁸), so parts are made in roughly the order they are installed.
Why it matters — and what it is not
The flagellar motor is central to microbiology far beyond swimming. Motility and chemotaxis are virulence factors: Helicobacter pylori uses flagella to burrow through stomach mucus, and motile Salmonella and Pseudomonas colonize hosts more aggressively. The flagellin filament is also a potent immune alarm — mammalian TLR5 and the cytosolic NAIP/NLRC4 inflammasome detect flagellin and launch inflammation, which is why some pathogens switch their flagella off inside a host.
A few persistent misconceptions are worth correcting: (1) The motor does not run on ATP — it runs on ion flux (H⁺ or Na⁺), like ATP synthase in reverse. (2) The filament does not beat or wave; it is a rigid corkscrew that is rotated. (3) Bacterial flagella are evolutionarily and structurally unrelated to eukaryotic cilia and flagella, which are built from microtubules and bend using the ATP-driven motor dynein — a case of convergent function, not shared parts. (4) The motor is not "irreducibly complex": its stator is homologous to the ExbBD/TolQR ion-driven systems, its export apparatus to the T3SS injectisome, and its ring proteins to other membrane assemblies, giving evolution abundant precursor modules to work from.
| Motor | Fuel | Motion | Speed | Torque / force |
|---|---|---|---|---|
| Flagellar motor (E. coli) | H⁺ motive force | Rotary, reversible | ~100–300 Hz | ~1,300 pN·nm |
| Flagellar motor (Vibrio) | Na⁺ motive force | Rotary, reversible | up to ~1,700 Hz | ~3,600 pN·nm |
| F₀F₁ ATP synthase | H⁺ motive force | Rotary (usually synthesizing) | ~130 Hz | ~40–50 pN·nm |
| Kinesin | ATP hydrolysis | Linear, along microtubule | ~100 steps/s | ~5–7 pN |
| Myosin II | ATP hydrolysis | Linear, along actin | ~10 nm/stroke | ~3–5 pN |
Frequently asked questions
What powers the bacterial flagellar motor if not ATP?
The motor is driven by the proton-motive force — the electrochemical gradient of H⁺ across the inner membrane, roughly −150 mV in E. coli. Protons flowing inward through the MotA₅MotB₂ stators are converted into rotation. Some bacteria, such as marine Vibrio, use a sodium-motive force instead, which lets their motors spin far faster.
How fast can the flagellar motor spin?
Proton-driven E. coli motors turn at about 100–300 Hz, or 6,000–18,000 rpm. Sodium-driven Vibrio motors reach roughly 1,700 Hz — over 100,000 rpm, faster than a jet turbine — because sodium ions cycle through the stator more quickly and can support higher torque.
How does the motor reverse direction so quickly?
The phosphorylated chemotaxis protein CheY-P binds the FliM subunits of the C-ring. When enough molecules bind, the whole ring cooperatively snaps into a clockwise conformation, changing which surface of the FliG ring the stators push. Because the switch is ultrasensitive (Hill coefficient near 10), the transition from counterclockwise to clockwise completes in under a millisecond.
How does a bacterium use the motor to find food?
Counterclockwise rotation bundles the flagella into a smooth 'run'; a clockwise flip makes the bundle fly apart, producing a 'tumble' that reorients the cell. By lengthening runs when conditions improve, the bacterium performs a biased random walk that carries it up nutrient gradients — a behavior called chemotaxis.
Is the bacterial flagellum the same as the flagellum on a sperm cell?
No — they only share a name. The bacterial flagellum is a rigid protein corkscrew spun by a rotary motor powered by ion flux. Eukaryotic flagella and cilia are bundles of microtubules that bend using the ATP-driven motor protein dynein. They are an example of convergent evolution, not common ancestry.
How does the flagellum build itself?
It self-assembles from the base outward using a type III secretion system. Protein subunits are exported unfolded through the hollow 2-nm central channel and add to the growing distal tip beneath a cap protein. A molecular ruler called FliK measures hook length and switches export from hook proteins to flagellin at the right moment.