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

The study found that living cells can move in ways that ordinary symmetry-based physics does not predict. Bacteria and human respiratory cells spontaneously choose curved paths that spiral in one direction. Their motion is also irreversible, meaning the reverse motion does not simply retrace the original route. This matters because it challenges models developed for passive liquids and organized materials.

The researchers observed this behavior while studying cellular flows and collective patterns. Swarming bacteria and flowing respiratory cells can organize into moving structures, then lose that organization. During these changes, individual cells follow curved, one-way spirals rather than mirror-symmetric paths. The active motion comes from cells using energy to push, swim, or flow.

The result broadens the physics of active matter. It shows that living systems can break mirror symmetry without an externally imposed direction. The article presents this as a challenge to conventional liquid-crystal physics, suggesting that future models must include energy-driven, irreversible cellular motion and changing collective patterns.

Mirror symmetry means that a shape, pattern, or motion looks unchanged when reflected across a mirror. Left and right are exchanged in the reflection. A perfectly straight path has no built-in left-right preference, but a curved spiral does. It winds with a particular handedness, either clockwise or counterclockwise.

The cells in the study spontaneously selected curved, one-directional spiral paths. A clockwise spiral reflected in a mirror becomes counterclockwise. Because the observed motion does not include both directions as equivalent alternatives, the living flow breaks mirror symmetry. The key point is that the cells create this preference themselves rather than merely following a visible external track.

This finding matters because symmetry is a powerful organizing idea in physics. Researchers often use it to simplify models and predict possible behavior. The article shows that active living matter can violate an expected mirror relationship while collective flows form and disappear. Such behavior requires models that capture cellular energy use, curved motion, and irreversible dynamics.

The study examined two scales of living matter. One was moving, single-celled bacteria that swarm together. The other was human respiratory cells moving within flowing tissue. The shared behavior is striking because these systems differ greatly in structure, size, and organization, yet both produced curved, irreversible spiral motion.

The source article does not provide exact dimensions for either cell type. As general biological context, many bacteria are roughly one to a few micrometers in length. Human respiratory cells are commonly around ten to several tens of micrometers across, depending on cell type and shape. These are typical ranges, not measurements reported by the excerpt.

The comparison suggests that symmetry breaking is not limited to one unusual organism or one size scale. It can emerge in free-swimming microbial groups and in more complex human tissue flows. That breadth is important for active-matter physics, though the precise mechanisms and dimensions may differ between bacteria and respiratory cells.

A reversible path can be imagined like a film played backward: the object returns along the same route in reverse order. At small scales, physicists often expect motion in simple fluids to have little memory and to obey strong symmetry constraints. Irreversible motion breaks that expectation because the forward and backward processes are not equivalent.

The studied cells moved in curved spirals with a fixed sense of turning. Reversing the motion would not reproduce the same track in the opposite direction. Cells actively consume energy to swim, push, and reorganize nearby material. Their collective flow can therefore generate effects that passive particles in a quiet liquid cannot create.

This is surprising because the behavior appears in both bacterial swarms and human respiratory-cell flows. It shows that irreversibility is not merely a detail of one biological system. It is part of how active living matter organizes itself. Physical models must therefore account for energy consumption, directional motion, and patterns that form and break down over time.

Liquid crystals are materials that can flow like liquids while their molecules keep a degree of shared orientation. This partial order produces direction-dependent behavior without making the material fully rigid. Physicists use liquid-crystal theory to describe moving patterns, alignment, defects, and changes in organized flows.

They are useful models because many cells also align and move collectively. A group of bacteria or tissue cells can form broad streams, vortices, or other patterns, much as oriented elements in a liquid crystal create structure. However, the study found an important difference. Cellular motion spontaneously curved into one-directional spirals and did not simply reverse.

That contrast challenges conventional liquid-crystal physics. Traditional models can describe organization and pattern formation, but living cells actively consume energy and generate forces. The article shows that these biological flows can break mirror symmetry and become irreversible. Liquid crystals remain a valuable starting point, but active living systems require additional terms for energy-driven motion and changing collective order.

Collective patterns form when nearby cells influence one another through motion and the forces they create. Bacteria swimming together can align into swarms and produce organized flows. Human respiratory cells can also move as a tissue, creating coordinated streams or other large-scale patterns. In both cases, many individual movements combine into visible collective behavior.

These patterns are not permanent. The flows can reorganize, weaken, or break down. During formation and breakdown, cells do not merely move along straight, mirror-symmetric routes. They follow curved paths with a preferred turning direction. Energy-consuming cellular activity drives the motion, while interactions among neighboring cells help transmit and reshape the flow.

The article’s central contribution is connecting pattern transitions with symmetry breaking and irreversibility. This means the beginning or collapse of order may involve more than a simple loss of alignment. It can reveal new, chiral motion at the cellular level. Understanding these transitions could improve descriptions of swarms, tissues, and other active biological materials.

Active matter consists of organisms or particles that consume energy to move and create flows. Earlier models often borrowed ideas from passive fluids and liquid crystals, where symmetry and reversibility provide useful constraints. The new study shows that living systems can escape those expectations while organizing collectively.

Bacteria and human respiratory cells formed and broke down collective flows, yet individual cells moved along curved, one-directional spirals. The spirals broke mirror symmetry because their reflected versions would turn the opposite way. They were irreversible because reversing time would not simply reproduce the same paths. These features arise from active cellular motion and interactions.

The consequence is a change in what physical models must explain. Models should include energy input, handed motion, non-reversible dynamics, and changing levels of order. This does not make liquid-crystal ideas useless; it shows where they are incomplete for living matter. The finding could guide future work on microbial swarms, tissue flows, and other systems that create motion from within.