The Bacterial Flagellar Motor Turns Out to Run on Meshed Gears

Harvard physicists measured a 6.2-to-1 gear ratio inside the flagellar motor and called it the first set of enmeshed gearwheels described in any living cell. The vocabulary is theirs, not ours: motor, rotor, stator, gears.

Published in PNAS, December 9, 2025 (open preprint on bioRxiv, October 2025). Summarized August 24, 2026.

What did the study add that we did not already know?

Biologists have long known the bacterial flagellum spins like a propeller. This work showed that each of the small torque-generating "stator" units driving it is itself an independent rotary motor. The researchers measured that inner rotation directly, inside living E. coli, rather than inferring it from static structures.

What is the "6.2 gear ratio," and why does it matter?

The ring of MotA proteins inside each stator unit turns about 6.2 times for every single turn of the large flagellar rotor, exactly the relationship two meshed gears of different sizes have. The authors describe the parts as meshing "like intermeshing gearwheels" and state this is the first set of enmeshed gearwheels described in any living cell. That is engineering, not analogy loosely applied.

How do you measure rotation inside something that small?

They used in vivo polarized photobleaching. A pulse of polarized light bleaches some of the fluorescent tags on the motor, and the polarization-dependent glow of the tags that remain reports their changing angle. Tracking that signal in tethered bacteria let the team clock how fast the inner ring turned relative to the whole motor.

Do the authors say anything about design?

No. The paper is a mechanistic account of how the motor generates torque. The engineering language is used because it is the most accurate way to describe what the molecule does, not to make any claim beyond the mechanism.