Spontaneous ordering of a bacterial drop into a spiral vortex

In this fluid dynamics video we explore the nature and causes of the spontaneous ordering that emerges in a dense bacterial suspension under confinement. Recent experiments with B. Subtilis confined within small flattened drops show that the bacteria form a steady single-vortex state with a counterrotating cell boundary layer. Using simulations that capture oriented cell-cell and cell-fluid interactions, we show that hydrodynamics is crucial in reproducing and explaining the phenomenon. We give new insights into the microscopic arrangement of the bacteria, which are confirmed by new experiments.