Orbiting of bacteria around micrometer-sized particles entrapping shallow tents of fluids
Hydrodynamics and confinement dominate bacterial mobility near solid or air-water boundaries, causing flagellated bacteria to move in circular trajectories. This phenomenon results from the counter-rotation between the bacterial body and flagella and lateral drags on them in opposite directions due to their proximity to the boundaries. Numerous experimental techniques have been developed to confine and maneuver motile bacteria. Here, we report observations on Escherichia coli and Enterobacter sp. when they are confined within a thin layer of water around dispersed micrometer-sized particles sprinkled over a semi-solid agar gel. In this setting, the flagellated bacteria orbit around the dispersed particles akin to planetary systems. The liquid layer is shaped like a shallow tent with its height at the center set by the seeding particle and the meniscus profile set by the strong surface tension of water. The tent-shaped constraint and the left handedness of the flagellar filaments result in exclusively clockwise circular trajectories. The thin fluid layer is resilient due to a balance between evaporation and reinforcing fluid pumped out of the agar. The latter is driven by the Laplace pressure caused by the curved meniscus. This novel mechanism to entrap bacteria within a minimal volume of fluid is relevant to near surface bacterial accumulation, adhesion, biofilm growth, development of bio-microdevices, and cleansing hygiene.