From swarming bacteria to tissue cells, living matter defies classic physical models of motion

A new study led by researchers from Ben-Gurion University of the Negev (BGU) reveals that living matter violates previously known physical principles of symmetry when cellular flows form and break down collective patterns. The research, published in Nature Physics, challenges conventional liquid-crystal physics by showing that moving single-celled bacteria and human respiratory cells spontaneously break mirror symmetry, moving in curved, irreversible spiral paths.

The collaborative study was led by Professors Avraham Be’er and Victor Yashunsky from BGU’s Jacob Blaustein Institutes for Desert Research (Sede Boqer Campus) and Department of Physics, alongside Professor Gil Ariel from Bar-Ilan University and Professor D. J. G. Pearce from the University of Geneva.

The puzzle of living ‘cowlicks’ and cellular swirls

In physics, collections of rod-shaped objects naturally align and flow together like liquid crystals. Bacteria and cells, which can move themselves, form a state known as an “active nematic.” Within these flowing swarms, natural disruptions called topological defects constantly form in pairs. While they look like the loops and whorls of a permanent fingerprint, these active twists are constantly moving, circulating through the swarm until they collide and cancel each other out.

Standard physical theories predict that in active nematic systems, these paired defects should approach and separate along straight, perfectly mirrored lines. However, by observing two vastly different biological systems across distant evolutionary boundaries—fast-swarming Bacillus subtilis bacteria and slow-crawling human bronchial epithelial cells—the researchers discovered that living systems break these physical expectations:

• Spontaneous mirror-symmetry breaking: Instead of following straight, symmetrical paths, defects veer off into distinct clockwise or counterclockwise trajectories, spontaneously breaking mirror symmetry.

• Persistent spiral paths: Rather than moving in straight lines, defect pairs travel toward and away from each other along curved spiral trajectories, actively co-rotating as they move.

• Irreversible biological time: Measuring entropy production revealed that the formation of a defect pair is not simply the movie of its destruction played backward. These creation and destruction events serve as major, irreversible drivers of energy dissipation in living systems.

A new ‘nemato-polar’ framework for biology

To bridge the gap between biological matter and synthetic physics, the team developed an active “nemato-polar” model. It accounts for how self-propelled cells burn chemical fuel to drive directional motion—introducing polar self-propulsion into a nematically ordered system.

When this directional self-propulsion is layered onto liquid-crystal alignment, it spontaneously creates an internal clockwise or counterclockwise twisting torque right at the core of the pattern disruption. This active rotational force pushes the defects into a permanent spiral trajectory from the moment they are born until they collide and disappear.

From physics to biofilms and wound healing

Understanding how living cells organize these collective flow patterns provides insight into fundamental biological processes. In nature, these physical dynamics govern how bacterial colonies coordinate to form resilient, drug-resistant biofilms and how epithelial tissue layers direct cellular traffic to close wounds during tissue repair.

“Our findings demonstrate that the self-propelling nature of individual cells fundamentally reshapes collective behavior at larger scales,” the researchers note. “By uncovering how microscopic motion imprints unexpected symmetries onto tissue and bacterial dynamics, this study introduces a new physical framework for understanding how living communities self-organize.”

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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