Bacterial swarms drive the propagation of active-passive boundaries through emergent vortical flows coupled to boundary curvature*

Many species of bacteria exhibit a collective behavior known as swarming, which features long-range self-organized flows on agar substrates. We found that swarming colonies of Serratia marcescens are capable of remodeling their physical environment by dissolving large domains of passive particles that obstruct the path of the expanding colony. Passive domains form when a swarm is exposed to high intensity light that locally immobilizes bacteria. Post-exposure, bacteria penetrate, shape, and erode the passive domain. We interpret the phenomena by identifying a propagating active-passive interface with an emergent interfacial stiffness generated by the action of vortical flows of bacteria colliding with the interface. Our results show that the evolution of the interface is governed by the local speed of bacteria coupled to the interface curvature, a result which suggests the existence of an active analogue to the Gibbs-Thomson-Stefan boundary in passive interphase systems. These observations broaden our understanding of swarming dynamics and provide insight into how bacteria compete for environmental niches permeated by particles such as dirt, spores, and other microbial species.