Swarming bacteria and human airway cells curl their topological defects into spirals

A nematic tracks alignment and little else: rod-shaped objects line up and flow together, and self-moving bacteria or cells in that state are called an active nematic ^[15]. Because a nematic does not care which end points forward, the textbook prediction comes out mirror-symmetric, with a defect pair approaching and separating along straight, perfectly mirrored lines ^[5]. Living cells do register direction. They burn chemical fuel and move the way they point, and when the group wrote that polar propulsion into a nematically ordered model, a twisting torque appeared at the core of the defect ^[9][10]. Clockwise or counterclockwise, it holds each defect on a spiral from the moment it forms until it collides with its partner and disappears ^[10].

The measurement spans two systems picked to be far apart: fast-swarming Bacillus subtilis and slow-crawling human bronchial epithelial cells ^[3]. That pairing makes the result hard to explain away. A swarming rod-shaped bacterium and a crawling epithelial cell share little mechanically, so an account built on one organism's propulsion hardware or one cell type's adhesions will not cover both ^[3]. They do share self-propulsion, the term the model adds ^[9].

The authors also measured irreversibility. Run the film of an annihilation backwards and you do not get a creation; entropy production quantifies that gap, and the researchers report defect creation and destruction as major irreversible drivers of energy dissipation in living systems ^[8].

The base for all of this is two biological systems, one bacterial species and one human cell type ^[16]. The phys.org account does not say how many defect pairs were tracked or how large any handedness bias was, and it describes no preparation that has nematic alignment without propulsion, and no observation inside an intact airway ^[17]. The model asks for that third experiment: switch off self-propulsion, keep the alignment, and the spirals should flatten back into the mirrored paths standard theory predicts ^[10][5]. The source describes distinct clockwise or counterclockwise trajectories, wording consistent with each pair choosing its own handedness ^[6].

The write-up goes further, to bacterial colonies coordinating into drug-resistant biofilms and epithelial layers directing cellular traffic to close wounds ^[11]. Those are the processes this physics is meant to inform. The team measured defect trajectories and entropy production in cell layers ^[7][8]. "Our findings demonstrate that the self-propelling nature of individual cells fundamentally reshapes collective behavior at larger scales," the researchers wrote ^[12]. The paper is "Irreversibility and symmetry breaking in the creation and annihilation of defects in active living matter," in Nature Physics ^[14].