Morphology and dynamics of growing bacterial swarm

In nature, bacteria commonly expand their territory through colony spreading, with the interface morphology of these colonies varying across different environments. While a swarming bacteria droplet growing in oil, it bears a resemblance to traditional viscous fingering patterns but is the opposite. The morphodynamics of such a growing bacterial swarm remains unclear. Here, we answer this question by using a continuum model of a droplet containing swarming-growing bacteria. On the one hand, bacterial motility always destabilizes the flat interface through random collision, which promotes fingering. On the other hand, cell growth favors a flat interface by eliminating short-wave interfacial fluctuations. Overall, during the nutrient-independence growth, the morphology of swarming-growing bacterial droplets is determined by the interplay among cell motility, cell growth, and interface pinning.