Confinement Discerns Swarmers from Planktonic Bacteria

Powered by flagella, many species of bacteria exhibit collective motion on a solid surface commonly known as swarming. Physical changes like cell elongation and hyper flagellation have been shown to accompany swarming phenotype. Less noticeable, however, are the contrasts of collective motion between the swarming cells and the planktonic cells of comparable cell density. Here, we show that when confined by microwells of specific sizes mounted on a soft agar surface, novel bacteria Enterobacter sp. SM3 under swarming condition exhibited "single-swirl" motion pattern distinct from "multi-swirl" motion pattern formed by its concentrated planktonic counterpart. We hypothesize that "rafting behavior" of the swarming bacteria upon dilution might account for the motion pattern difference. This was further validated through computational simulation where swarming cells are modeled by lower repulsion and stronger alignment among them than planktonic cells. Our new technical approach also enabled us to observe swarming on a non-agar tissue surface.