Living cells break a symmetry rule that liquid crystals were supposed to obey

Swarming bacteria and layers of human respiratory-tract cells move in curved, one-directional spiral paths as their collective patterns form and collapse — a pattern that classic liquid-crystal physics says shouldn’t happen, according to a study reported by Phys.org. The work, led by researchers at Ben-Gurion University of the Negev and published in Nature Physics, found the same unexpected behaviour in two very different kinds of living matter: single-celled bacteria and human respiratory cells.

The model that assumed even-handedness

Liquid-crystal theory has been the workhorse for describing “active matter” — flocks of bacteria, cell sheets, bird flocks, anything made of many self-propelled units that flow like a fluid but retain some internal order. A core assumption in that theory is mirror symmetry: a swirl spinning clockwise should be just as likely, and just as reversible, as one spinning anticlockwise. Nothing in the mathematics should prefer one handedness over the other.

What the Ben-Gurion team measured was cellular flows nucleating vortices and then breaking them down again, over and over — and finding that both the bacterial suspensions and the tissue-cell monolayers consistently favoured one spin direction as those patterns formed and dissolved. That is a violation of the assumed mirror symmetry, and it means the motion is not just complex but irreversible in a specific sense: you cannot simply run the tape backwards and get an equally valid version of events, as the standard theory implies you should.

Why the mechanism matters more than the observation

The wire report does not say what causes the handedness — whether it traces back to the chirality of individual bacterial flagella, some asymmetry in how tissue cells crawl and divide, or something else entirely. That is the open question the paper leaves for the next round of work.

What it does establish is that a mathematical shortcut long borrowed from equilibrium physics to describe biological flows carries an assumption that active, living systems do not honour, at least at the moments when collective patterns are being built or torn apart. Those transition moments are exactly the ones with biological stakes — a biofilm reorganising, an epithelial sheet closing a wound, cells migrating during development or in disease. If models built on an assumption of mirror symmetry misdescribe those transitions, then predictions drawn from them — about how fast a wound heals, or how a bacterial colony spreads — may be built on the wrong footing precisely where it counts most.

Reported at Phys.org; analysis ours.