🧫 Bacteria swim using a tiny electric motor that spins several hundred times per second

Image of the motor by Prash Singh.

• Researchers have now mapped every part of the flagellar motor that bacteria use to swim, after more than 50 years of research.
• The motor spins several hundred revolutions per second and is powered by protons streaming into the cell at more than 2,000 per second.
• Mike Manson, professor emeritus of biophysics at Texas A&M University, began studying the flagellar motor in the 1970s. After 50 years, he now understands how it works.

50 years of research on a molecular motor

The flagellar motor is an electric motor that single-celled bacteria use to move toward food. It spins a tail-like flagellum that pushes the cell forward. When the motor rotates counterclockwise, the cell swims straight ahead at ten times its own body length per second or more. When it rotates clockwise, the cell tumbles around and changes direction.

The motor was discovered in the 1970s by biophysicist Howard Berg at Harvard. He built an automatic tracking microscope that could follow a single bacterium, and found that bacteria alternate between swimming straight and tumbling. He also proposed that the motor actually rotates — an idea considered impossible in biology at the time.

Cryo-EM provided the answers

Improvements over the past 15 years in the imaging technique cryo-electron microscopy have made it possible to see the motor's parts. At the base sits a C ring of 34 identical proteins. Above the ring sit smaller protein complexes called stators. E. coli has 10 to 12 stators available per flagellum, and the number that lock into the C ring depends on the weight of the cell or the viscosity of the surrounding fluid.

Each stator consists of two central proteins surrounded by five proteins that form a pentagonal ring. This 5:2 geometry was mapped in 2020 by two independent research groups: one at the University of Oxford led by Susan Lea, and a joint group from the University of Copenhagen and Humboldt University of Berlin.

Protons drive the rotation

The pentagonal ring turns like a turnstile, one-tenth of a revolution at a time. What drives it is a stream of protons flowing into the cell. More than 2,000 protons pass through each pentagonal turnstile every second. In December 2025, Aravinthan Samuel at Harvard published results confirming this mechanism.

Inside the bacterium there are fewer than 100 free protons at a time, while a similar volume of water outside the cell contains tens of thousands. The difference is maintained by electron transport chains that pump out thousands of protons per second. It was biochemist Peter Mitchell who proposed this proton motive force in 1961, an idea that earned him the Nobel Prize in Chemistry in 1978.

The direction switch has been mapped

Two studies from 2024, one led by Lea at the National Institutes of Health and one led by Tina Iverson at Vanderbilt University, showed how the motor switches direction. When the bacterium detects fewer nutrients, proteins called CheY become phosphorylated. Within milliseconds, a phosphorylated CheY molecule binds to one of the C-ring proteins. That causes the entire ring to change shape, like a hair clip snapping into its other position.

In the new configuration, the stators rotate against the inner edge of the C ring instead of the outer edge, causing the C ring to spin clockwise. The flagellar bundle falls apart and the cell tumbles. When the phosphorus atom falls off, the ring returns to its original form. A study from Samuel's group in March 2026 confirmed that the system responds to a single signaling molecule.

Mike Manson, professor emeritus of biophysics at Texas A&M University, began studying the flagellar motor in the 1970s. After 50 years, he now understands how it works.

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