Capillary spreading governs the formation and healing of holes in Myxococcus xanthus monolayers

Colonies of the self-propelled, rod-shaped bacterium Myxococcus xanthus, grown on hydrogel substrates, form an active matter system with nematic order and short-ranged attractive forces mediated by capillarity. We previously showed that holes in dense monolayers nucleate preferentially at –1/2 topological defects, consistent with the flow fields predicted for dry active nematics. Here we investigate the physical mechanisms that control the nucleation, growth, and healing of these holes. Using surface profilometry, we find that holes emerge from large local fluctuations in cell velocity that rupture the thin capillary bridge spanning the monolayer. We further show that holes heal by capillary-driven closure, with spreading dynamics governed by the presence and geometry of remaining liquid bridges. These observations support a view of the coupled cell–fluid system as an active thin film that undergoes spontaneous cavitation and capillary spreading, suggesting a new framework for describing topological defect dynamics in active monolayers.