Questions: Bacterial Flagellar Motor and Rotation Mechanics

Question 1

A bacteriologist treats E. coli with a drug that collapses the proton gradient across the inner membrane without affecting intracellular ATP levels. What effect would you predict on flagellar motility?

• A. Motility continues normally because the flagellar motor runs on ATP, not proton flow
• B. Motility slows slightly but is maintained by the remaining membrane potential
• C. Motility stops because the flagellar motor is powered by proton-motive force, and eliminating the proton gradient removes its energy source
• D. The bacterium switches to using NADH directly to power the motor as a backup energy source

Answer: (C)

The bacterial flagellar motor is directly powered by the proton-motive force (PMF) — the electrochemical gradient of protons across the inner membrane — not by ATP hydrolysis. Protons flow through the stator complex (MotA/MotB channels) and their passage exerts force on the rotor's FliG ring, generating torque. Collapsing the proton gradient eliminates the driving force for this ion flow, stopping the motor entirely even if ATP levels are unaffected. This contrasts with eukaryotic motor proteins like myosin and kinesin, which are ATPases and would be inhibited by ATP depletion rather than by ion gradient collapse.

Question 2

When E. coli flagellar motors switch from counterclockwise (CCW) to clockwise (CW) rotation, what happens to the bacterium's movement and why?

• A. The bacterium swims faster in the forward direction because CW rotation is more efficient
• B. The bacterium reverses direction smoothly, like a car shifting into reverse
• C. The bacterium tumbles randomly because CW rotation causes the flagellar bundle to fly apart, disrupting coordinated propulsion
• D. The flagella retract into the cell during CW rotation to prepare for a new direction

Answer: (C)

In E. coli, multiple flagella bundle together into a coherent helical propeller only when all motors spin counterclockwise. This bundle pushes the cell forward in a 'run.' When one or more motors switch to clockwise rotation, the flagella cannot maintain the bundle geometry — they fly apart and the cell tumbles, reorienting randomly. When the motors switch back to CCW, the flagella rebundle and the cell swims forward in a new direction. This run-and-tumble mechanism is how bacteria navigate chemical gradients (chemotaxis): they tumble less frequently when moving toward attractants, and more frequently when moving away.

Question 3

The bacterial flagellar motor is described as a 'true rotary motor' because the entire flagellar filament rotates continuously around its own axis relative to the cell body.

• T. True
• F. False

Answer: True

Unlike eukaryotic motor proteins (myosin, kinesin, dynein), which are linear motors that generate force by stepping along a track, the bacterial flagellar motor produces continuous rotational torque. The rotor spins relative to the stator, and this rotation is transmitted through the hook to the flagellar filament, which rotates like a propeller. This is genuine rotation — the filament turns continuously, not in a reciprocating back-and-forth motion. This is remarkable because most biological motion-generating systems produce linear forces; the flagellar motor is one of the very few true biological rotary engines.

Question 4

The bacterial flagellar motor operates on the same basic principle as eukaryotic motor proteins: it hydrolyzes ATP to drive conformational changes that generate mechanical force.

• T. True
• F. False

Answer: False

The bacterial flagellar motor uses proton-motive force, not ATP hydrolysis. Protons flow down their electrochemical gradient through the MotA/MotB stator channels, and this ion flow drives conformational interactions between the stator and the FliG ring of the rotor, generating torque. ATP is not the direct energy currency of the motor. This makes the flagellar motor mechanistically distinct from myosin (ATP hydrolysis + actin filament), kinesin (ATP hydrolysis + microtubule), and dynein (ATP hydrolysis + microtubule). Some bacteria use sodium ions instead of protons, further demonstrating that ion gradients — not ATP — are the operating principle.

Question 5

Explain why the bacterial flagellar motor is considered a 'true rotary engine' and how this distinguishes it mechanistically from the motor proteins found in eukaryotic cells.

Model answer: The bacterial flagellar motor generates continuous rotational torque: the rotor (including the C ring with FliG, FliM, FliN) spins relative to the stator (MotA/MotB) embedded in the cell membrane, and this rotation is transmitted through the hook to the flagellar filament, which turns like a propeller. The energy source is proton-motive force — protons flowing through stator channels drive the rotation by exerting successive forces on the rotor ring. Eukaryotic motor proteins (myosin, kinesin, dynein) are linear motors: they hydrolyze ATP to produce conformational changes that generate linear stepping along cytoskeletal tracks (actin or microtubules). They do not rotate; they walk. The flagellar motor is one of the only true biological rotary engines, operating on an entirely different principle.

The distinction matters for understanding the diversity of biological machines. Linear motors convert chemical energy into linear mechanical displacement; the flagellar motor converts electrochemical energy (ion gradient) directly into rotational motion. The motor can also reverse direction in milliseconds (by switching the interaction geometry between stator and rotor), dynamically recruit additional stator units to increase torque, and achieve speeds exceeding 1,000 revolutions per second — feats that have no direct parallel in the eukaryotic cytoskeletal motor repertoire.