Rotational dynamics of bound pairs of bacteria-induced membrane tubes.

We present experiments demonstrating tube formation in giant unilamellar vesicles that are suspended in a bath of swimming E. coli bacteria. We chose the lipids such that the bacteria form no adhering interactions with the membrane. The tubes are generated by the pushing force exerted by the bacteria on the membrane of the vesicles. Once a tube is generated, the bacterium is confined within it, resulting in long-lived tubes that protrude into the vesicle. We show that such tubes interact to form stable bound pairs that orbit each other. We speculate that the tubes are maintained by the persistent pushing force generated by the bacteria, and the rotating pairs are stabilized by a combination of curvature-mediated interaction and vorticity generated in the membrane by the rotation of the flagella.