Orbiting of bacteria around micrometer-sized particles entrapping shallow tents offluids*

We report discovery of orbital motion of flagellated bacteria when they are confined within a thin layer of water around dispersed micrometer-sized particles sprinkled over a semi-solid agar gel. The liquid layer is shaped like a shallow tent with the height at the center set by the seeding particle and the meniscus profile set by the strong surface tension of water. The thin fluid layer is resilient against evaporation due to agar substrate serving as an enduring reservoir. The tent-shaped constraint and the left handedness of the flagellar filaments result in exclusively clockwise circular trajectories when experiments were done with E. coli and Enterobacter sp. 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.