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 described it as the first set of meshed gearwheels found in any living cell. The machine at the heart of the irreducible-complexity debate just gained another moving part.

Published in PNAS on December 9, 2025, with an open preprint on bioRxiv from October 2025.

What did the study add?

Biologists have long known the bacterial flagellum spins like a propeller. This work found that each of the small units driving it is itself a rotary motor, and it measured that inner rotation directly inside living E. coli rather than inferring it from still images.

What is the 6.2 gear ratio?

The ring of proteins inside each drive unit turns about 6.2 times for every single turn of the large flagellar rotor, the same relationship two meshed gears of different sizes share. The researchers describe the parts as intermeshing gearwheels and call this the first set of meshed gears found in a living cell.

How do you measure rotation at that scale?

They used a technique that tags the motor with fluorescent markers, bleaches some of them with a pulse of polarized light, then watches how the glow of the remaining markers changes as they turn. Tracking that signal in living bacteria let the team clock the inner ring against the whole motor.

Why does the flagellum come up so often in this debate?

It has become the standard example of a system whose many precisely matched parts all seem necessary for it to work at all. Each new layer of mechanism, now including gearing, adds parts that have to fit together for the motor to turn.