Microbial hockey: ISTA scientists discover how bacteria rotate tiny pucks and create unusual materials
Sparks fly. A hammer repeatedly strikes an anvil. Hit after hit, a red-hot piece of metal slowly takes the shape of a sword. The blacksmith, satisfied, holds it up in the air before returning it to the blazing oven to reheat it until it glows. This scene could easily belong in a medieval fantasy series such as The Witcher or video games such as Elden Ring.
At ISTA, ERC grantee Jérémie Palacci’s research group is venturing into metallurgy, albeit with a twist. Instead of traditional tools, the scientists use E. coli bacteria, often associated with infections linked to contaminated food. When placed in water, their long flagella, tail-like stuctures that propel them,create a so-called active bath. This dynamic environment helps form gel-like aggregates by acting like a small fire and raising the ‘temperature’ to an equivalent of 2000 °C, similar to what a blacksmith needs to craft metals. It can even spin tiny micro discs.
In their new joint publication, ISTA’s Daniel Grober and Jérémie Palacci, along with Tanumoy Dhar and David Saintillan from the University of California, San Diego, reveal the process behind this discovery. They conducted their research in the Materiali Molli Lab, which is located on ISTA’s campus, in collaboration with the Department of Physics at UC San Diego.
Micro rotors
In a 2023 Nature Physics publication, Palacci, Grober and colleagues demonstrated that these bacteria-fuelled active baths propel sticky colloids (round beads that stick together when in contact) into rotating aggregates driven by the spin of E. coli flagella, though the mechanism remained unclear.
Inspired by a 2010 study showing that bacteria only rotate asymmetric gears, the team initially attributed this behaviour to shape asymmetry. However, the irregular structure of the clumps made this difficult to verify experimentally (Video 1).
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E.coli rotate
The researchers speculated that the asymmetrical shape might also be the cause of their rotating clumps.
© Palacci Group/Nature Physics
‘In this work, bacteria acted like tiny vehicles, constantly nudging the asymmetric gear to spin,’ Palacci theorised at the time.
Spinning ‘hockey pucks’
To overcome this, the researchers simplified the system by creating smooth, symmetrical micro-discs using a 3D nanoprinter. When placed in E. coli active baths, these discs unexpectedly rotated clockwise, disproving the idea that asymmetry is required (Video 2).
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E.coli swim around puck
© Grober et al./Nature Physics
More complex discs with internal compartments rotated faster, as confinement allowed bacteria to act like paddles. Even open structures rotated as soon as a single bacterium passed through, indicating that direct contact with surfaces is not necessary (Videos 3 and 4).
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E.coli power discs
© Grober et al./Nature Physics
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Single E. coli swimming
© Grober et al./Nature Physics
Hydrodynamic interaction
Palacci explains that direct contact is not the key to spinning the discs. This is unlike what had been observed with asymmetrical gears. Instead, swimming E. coli generate torque by counter-rotating their bodies and flagella, which twists the surrounding fluid.
Although these flows cancel at the centre, they act at different points along the chamber, creating a net rotational force that spins the disc. Mathematical models support this mechanism. ‘It is a well-known result in our field that the counter-rotation of the body and flagella (tail) of an E. coli cause it to swim in clockwise circles near a solid surface,’ Grober explains.
‘We realised that we could flip these dynamics upside down by confining the E. coli in a microscopic channel beneath the puck. These experiments utilise the exact same hydrodynamic effect to create, essentially, a microscopic and contactless engine, which drives the persistent rotation of the puck.’
Impact on medical therapeutics & sustainability?
This is an important finding because the effect depends on confinement, is cumulative, and largely independent of object shape, suggesting it may be widespread in natural environments, from biofilms to soils.
‘Despite its significance, this effect has been overlooked until now,’ Palacci says. ‘We hope that this novel understanding will have a meaningful impact on medical therapeutics or sustainability efforts.’
Publication:
Grober et al. 2026. The hydrodynamic torque dipole from rotary bacterial flagella powers symmetric discs. Nature Physics. DOI: 10.1038/s41567-026-03189-4
