Fluid motion at the leading edge of an E. coli swarm

A microbubble tracer (bright spot, 2.2 μm diameter) moved downwards 74.5 μm while the swarm advanced 17.0 μm. This reveals rapid clockwise flow of fluid at the leading edge of the swarm, driven by flagellar rotation. See: Wu et al., PNAS 108(10): 4147-51.