  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.
