Sparks fly. A hammer repeatedly strikes an anvil. Hit after hit, the 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 like Elden Ring.
At the Institute of Science and Technology Austria (ISTA), 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 infection linked to contaminated food. When placed in water, their long flagella—tails that propel them forward—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 one a blacksmith needs to craft metals. It even manages to 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-fueled active baths successfully propel sticky colloids—round beads that stick together when in contact—to form gel-like aggregates that rotate clockwise, originating from the clockwise spin of the E. coli flagella. However, the reasoning behind this behavior was unclear.
Inspired by a 2010 study where bacteria interacting with gears—both symmetric or asymmetric—caused only asymmetrical gears to turn, Grober and Palacci investigated further.
"In this work, bacteria acted like tiny vehicles, constantly nudging the asymmetric gear to spin," Palacci theorized at the time.
The researchers speculated that the asymmetrical shape might also be the cause of their rotating clumps (Video 1). However, measuring this effect proved challenging, as the random asymmetry in the clumps led to too much data noise.
Spinning 'hockey pucks'
The scientists, therefore, needed to take a step back and come up with an experiment to clarify what was happening. To do so, Grober used a 3D nanoprinter to create smooth, symmetrical micro discs similar to hockey pucks. After introducing these 'pucks' into the active baths filled with E. coli, they were surprised to see them spin clockwise, which negated the earlier hypothesis that symmetrical shapes do not turn (Video 2).
The researchers also found that a slightly more detailed puck with, for example, four compartments extending toward the center, spun even faster than its more basic counterparts (Video 3). The confined spaces allowed the bacteria to act like tiny paddles, enhancing the spin. Interestingly, a single-compartment puck with no closed end rotated as soon as one E. coli swam through implying that mechanical contact with the wall of the chamber was not necessary for its movement (Video 4).
Hydrodynamic interaction
Palacci clarified that direct contact is not the key to spinning the discs. This is unlike what had been seen with asymmetrical gears. Instead, the new study shows that swimming E. coli twist the fluid around them simply because of how they swim. Their bodies rotate in one direction while the flagella spin in the opposite.
This twisting action, or torque, causes the liquid to swirl both in front of and behind the swimming E. coli, creating a traction force on the top wall of the chamber. Even though these rotational movements cancel each other out and the center of the puck remains stable, an overall torque is created that causes the puck to spin. This is because the rotations occur at different points along the chamber.
Think of trying to open a jam jar by trying to twist the lid off, but the center does not budge. Mathematical models aligned with these observations, offering evidence that E. coli drive motion through hydrodynamic interaction.
"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 realized that we could flip these dynamics upside down by confining the E. coli in a microscopic channel beneath the puck. These experiments utilize 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 ability of flagellated bacteria to rotate objects relies on confinement and is cumulative and agnostic to the shape of the object it rotates. Essentially, this phenomenon should be observable whenever bacteria are in tight spaces—a common occurrence in nature, whether within biofilms, which are crucial to bacterial resistance, or in soils, where bacteria play a vital role in maintaining ecosystem balance.
"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."
