Spontaneous formation of active nematic droplets in bacterial swarm

Active system consists of units that consume energy to move or exert mechanical forces, driving the system far away from equilibrium and arising remarkable behaviors. One popular feature of active matter is collective motion behavior which forms the pattern with a strikingly larger scale than individuals. Long active molecules endow intrinsic orientational order, presenting active nematic system. Spatially confining active units in a volume of fluid has the potential to design a self-propulsion system by mutual interaction between boundary and active force.

Here, we introduced two novel types of biological collective motion in a bacterial swarm consisting of densely packed aligned Bacillus subtilis cells: “propeller” with persistent motion mode and one +1/2 topological defect; “rotor” with rotating motion mode and two +1/2 topological defects. We clarified that the formation of multicellular clusters resulted from the collective motion of rod-like bacteria and the rotating chirality comes from the hydrodynamic interactions between the rotating flagellar and substrate. Moreover, our phase field continuum active nematic simulation reproduced the “rotor” and “propeller” modes in dense bacterial systems. In addition, we discovered that elongated bacteria-formed propellers gain high collective tolerance when encountering stress, which may have potential applications in new antibacterial approaches.