By conventional thinking, bacterial swimming results in negligible flows, and thus diffusion effects dominate over stirring. But Goldstein and colleagues took advantage of the well-known phenomenon of bioconvection in which, in a drop lying on a surface, bacteria are moving cooperatively toward the more oxygenated upper layer. After concentrating in this layer they are subject to gravity and stochastically form downward plumes. Once started, these plumes feed themselves: the incompressibility of water means that falling bacteria draw in more bacteria behind themselves.The researchers discovered a new phenomenon near the corner of the drop, where the fluid meets the supporting solid. Here, the curved drop surface coincides more and more with the direction of gravitational pull, so falling bacteria are channeled along the surface of the drop, driving a circulating vortex. These parcels of liquid thus spend considerable time by the surface, where they pick up hefty doses of oxygen before diving back toward the deoxygenated center of the drop. “It's a little conveyor belt,” says Goldstein. “It's a very efficient mixer.”
The team visualized the flows both in vitro and in silico, although the oxygen measurements are yet to be made in vitro. The flows would be expected to affect the spreading of other metabolites, including quorum signals that prompt biofilm formation.