Biophysical Society Thematic Meeting | Ascona 2026

Mechanobiology of Infection

Wednesday Speaker Abstracts

COLLECTIVE BEHAVIOR AND ACTIVE PHASE TRANSITIONS IN MOTILE BACTERIAL POPULATIONS Joshua Shaevitz Princeton University, Physics, Princeton, NJ, USA Myxococcus xanthus is a soil bacterium that exhibits striking forms of collective behavior during both growth and starvation. Large populations of cells move across surfaces using gliding motility and interact through steric collisions, mechanical alignment, and periodic reversals of direction. These local interactions generate emergent populat ion level dynamics that closely resemble active phase behavior in driven soft matter systems.In this talk I will present recent experimental and theoretical work that uses M. xanthus as a model system for studying active phase transitions in living materials. High resolution microscopy and quantitative analysis reveal that dense populations form nematically ordered states with mobile topological defects that organize large scale flows within the colony. Mechanical stresses and cell reversals regulate the creation and motion of these defects and shape the evolving spatial structure of the population .I will also discuss how the collective state of the colony changes as environmental conditions vary. During growth, populations behave as an active gas of motile cells that form transient polar domains. Under starvation, cells modulate their motility and interactions in ways that effectively tune the collective Péclet number of the system, driving a transition toward aggregated droplet like structures that precede fruiting body formation. During predation, colonies exhibit traveling surface waves that arise from an activity-driven instability associated with cell reversals and collisions. Together these results illustrate how bacterial populations can realize a rich set of active matter phases and provide a quantitative framework for connecting single cell motility with emergent multicellular organization.

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