A new study of swarming bacteria and human bronchial epithelial cells has uncovered a hidden asymmetry in the microscopic events that shape living matter: the birth and death of topological defects do not unfold as mirror images of one another. Although the two biological systems operate at dramatically different scales and perform entirely different functions, both generate the same distinctive pattern of disorder. Their defect pairs possess half-integer charges, a signature associated with nematic organization, yet the dynamics that create and destroy those defects violate the spatial mirror symmetry expected from conventional descriptions. The finding suggests that a fundamental feature of life—the ability to generate directed forces while organizing collectively—can make biological materials intrinsically irreversible. In the emerging field of active matter, this offers a striking example of how living systems depart from the rules that describe passive fluids, crystals and liquid crystals. It also identifies defect formation and annihilation as potentially important sources of entropy production, linking the choreography of cells and microbes to the thermodynamics of nonequilibrium physics.
Topological defects are not ordinary particles or holes. They are localized disruptions in an otherwise ordered field, points or lines around which the direction of neighboring elements rotates in a particular way. In a liquid crystal, for example, molecules may align along a common axis, but that axis has no inherent arrow: pointing north or south describes the same orientation. This head-tail symmetry is known as nematic order. When the orientation field becomes impossible to define smoothly, a defect appears. The amount and direction of rotation around it determine its topological charge. Half-integer defects are especially important in active nematics because they can behave differently from one another: some have a sharply pointed, comet-like structure, while others are more geometrically balanced. In passive materials, such defects can be created or removed through processes that are approximately reversible when viewed at the microscopic level. In living matter, however, the constituents consume energy and produce forces continuously, driving the system away from equilibrium. That energy consumption changes the rules governing how order breaks down and reforms.
The researchers examined two forms of active living matter that could hardly seem more different. One consisted of swarming bacteria, whose collective motion emerges from the interactions of many tiny organisms. The other involved human bronchial epithelial cells, which form living tissues lining the airways. Bacteria and epithelial cells differ in evolutionary history, dimensions, biological purpose and the mechanisms through which they move. Yet both displayed half-integer topological defects in their collective orientation fields. This commonality is significant because it indicates that the observed behavior is not tied to one species, one molecular motor or one particular biological architecture. Instead, it may arise from a general physical tension inside active organisms: their structures can be organized in a nematic fashion while their internal activity generates polar forces with a preferred direction. The first feature makes the defects resemble those of an active nematic, whereas the second supplies a directional bias that standard nematic models do not fully capture.
The crucial discovery emerged when the scientists compared the geometry of defect creation and annihilation. When a pair of defects forms, the line connecting them provides a natural axis against which the surrounding motion can be measured. If the process preserved mirror symmetry, the flow on one side of that line would have a corresponding reflection on the other. Instead, the researchers found that the creation and disappearance of defect pairs spontaneously selected one side. The pattern was not simply a rotated version of a symmetric event; it was intrinsically chiral-like in its spatial organization, even though the underlying nematic structure itself does not distinguish left from right. The asymmetry means that the pair’s appearance or destruction carries information about directional forces generated by the living material. A defect event therefore cannot be understood only as the rearrangement of orientations. It is also a force-generating process, in which active motion chooses a spatial pathway and breaks a symmetry that passive theory would normally preserve.
This result challenges a common simplifying assumption in the physics of active nematics. The term “nematic” describes the symmetry of the alignment field, but it does not necessarily describe every force acting within the material. In a textbook nematic, the local orientation is an axis without a head or tail. Biological agents, by contrast, often push, pull, crawl or swim with a physical polarity. A bacterium propels itself in a direction; a cell can extend a front and retract a rear; molecular motors can transport material along filaments. The study proposes that the observed symmetry breaking arises from a dualism between these two properties. The collective arrangement remains nematic at the level of orientation, while the forces generated by the constituents are polar. When defects are created or annihilated, polar activity can couple to the nematic field, selecting a preferred side of the defect pair. This hybrid organization may be a defining feature of living active matter rather than a minor correction to existing models.
The distinction between creation and annihilation is equally important. In a reversible process, recording a system’s evolution forward in time and then reversing the sequence would reproduce the original trajectory. At equilibrium, microscopic dynamics can often be treated in this way, at least in principle. But the researchers’ analysis indicates that defect creation and defect annihilation are not reverse versions of the same event. They produce different spatial signatures and different levels of entropy production. Entropy production is a measure of irreversibility: it quantifies how much energy is dissipated as organized activity drives a system through states that cannot simply be retraced. For active organisms, the energy ultimately comes from metabolism. Chemical energy is converted into motion, stresses and rearrangements, while some of it is lost as heat. The results imply that the defect-mediated part of this process is not passive bookkeeping. The events through which order is born and erased may be major sites where biological energy consumption becomes physically visible.
That conclusion gives topological defects a broader role in understanding living systems. Defects are often treated as transient disturbances in an otherwise coherent field, but in active matter they can organize motion over larger distances. A defect may attract, repel or propel surrounding material, and a population of defects can influence how a tissue flows or how a microbial swarm rearranges itself. If their creation and annihilation are asymmetric, then defects do more than mark where alignment fails. They become engines of directed change, helping convert local biological activity into collective transport and remodeling. In an airway epithelium, coordinated cell motion is relevant to the maintenance and repair of a protective barrier. In bacterial communities, collective flows can influence how organisms spread and reorganize. The study does not claim that the same biological function is controlled by defects in both systems, but their shared physical behavior suggests that nonequilibrium principles may connect processes across very different forms of life.
The findings may also matter for how scientists interpret the thermodynamics of tissues and microbial communities. Many theoretical descriptions of active matter begin with idealized particles or simplified alignment rules, allowing researchers to isolate broad principles. Such models have successfully explained how collective motion can arise without a central controller. Yet the new work suggests that models based only on nematic symmetry may overlook the directional forces generated by real organisms. To account for the observed behavior, theories may need to include the coupling between an orientation field with head-tail symmetry and force-producing agents with a front-back distinction. That coupling could alter predictions for defect lifetimes, pair trajectories, energy dissipation and the large-scale flow of the material. It may also help distinguish living active matter from engineered systems, where energy input can be programmed but does not necessarily arise from the adaptive, internally coordinated machinery of cells.
The study’s most provocative implication is that irreversibility can be read from geometry. Rather than measuring only how fast bacteria or cells move, researchers can examine the shape and orientation of defect events to infer how energy flows through a living material. A microscopic pattern around a defect may therefore serve as a fingerprint of hidden biological activity. Because the systems investigated span bacteria and human cells, the effect could inspire searches for similar asymmetries in other active materials, including developing tissues, cellular layers, synthetic swimmers and dense microbial communities. It could also provide a framework for comparing organisms that have no obvious biological similarity but share a physical strategy: arranging themselves collectively while continuously generating force. The work does not replace the nematic description; it exposes its limits. Living matter can look nematic in structure while behaving polar in motion, and that mismatch may be precisely where its irreversibility emerges.
By showing that defect pairs are created and destroyed through different, mirror-symmetry-breaking pathways, the researchers have placed a new constraint on theories of active biological matter. The message is both technical and visually intuitive: when living systems reorganize, they do not merely shuffle their parts back and forth. They consume energy, generate directional forces and leave behind a record of that activity in the geometry of collective motion. Swarming bacteria and human airway cells thus become two examples of a wider physical principle in which life’s ability to act introduces a preferred direction into structures that would otherwise be symmetric. Topological defects, once viewed mainly as mathematical singularities or transient imperfections, may consequently be among the clearest places to observe the thermodynamic cost of being alive. Their asymmetric birth and death reveal that the microscopic mechanics of living matter cannot be fully captured by borrowing equilibrium concepts alone.
Subject of Research: Irreversibility and symmetry breaking in topological defect creation and annihilation in active living matter
Article Title: Irreversibility and symmetry breaking in the creation and annihilation of defects in active living matter
Article References: Be’er, A., Neimand, E. D., Agarwal, Y., Corbett, D., Pearce, D. J. G., Ariel, G., & Yashunsky, V. (2026). Irreversibility and symmetry breaking in the creation and annihilation of defects in active living matter. Nature Physics. https://doi.org/10.1038/s41567-026-03378-1
Image Credits: AI Generated
DOI: 10.1038/s41567-026-03378-1
Keywords: active matter, topological defects, bacterial swarms, epithelial cells, nematic order, symmetry breaking, entropy production, nonequilibrium physics
