Large-Scale Vortices with Dynamic Rotation Emerged from Monolayer Collective Motion of Gliding Flavobacteria.

A collective motion of self-driven particles has been a fascinating subject in physics and biology. Sophisticated macroscopic behavior emerges through a population of thousands or millions of bacterial cells propelling itself by flagellar rotation and chemotactic responses. Here, we found a series of collective motions accompanying successive phase transitions for a nonflagellated rod-shaped soil bacterium, Flavobacterium johnsoniae, which was driven by a surface cell movement known as gliding motility. When we spotted the cells on an agar plate with a low level of nutrients, the bacterial community exhibited vortex patterns that spontaneously appeared as lattice and integrated into a large-scale circular plate. All patterns were exhibited with a monolayer of bacteria, which enabled us to two-dimensionally visualize an individual cell with high resolution within a wide-range pattern. The single cells moved with random orientation, but the cells that were connected with one another showed left-turn-biased trajectories in a starved environment. This feature is possibly due to the collision of cells inducing a nematic alignment of dense cells as self-propelled rods. Subsequently, each vortex oscillated independently and then transformed to the rotating mode as an independent circular plate. Notably, the rotational direction of the circular plate was counterclockwise without exception. The plates developed accompanying rotation with constant angular velocity, suggesting that the mode is an efficient strategy for bacterial survival. IMPORTANCE Self-propelled bacteria propelled by flagellar rotation often display highly organized dynamic patterns at high cell densities. Here, we found a new mode of collective motion in nonflagellated bacteria; vortex patterns spontaneously appeared as lattice and were integrated into a large-scale circular plate, comprising hundreds of thousands of cells, which exhibited unidirectional rotation in a counterclockwise manner and expanded in size on agar. A series of collective motions was driven by gliding motility of the rod-shaped soil bacterium Flavobacterium johnsoniae. In a low-nutrient environment, single cells moved with random orientation, while cells at high density moved together as a unitary cluster. This might be an efficient strategy for cells of this species to find nutrients.