Confinement Discerns Swarmers from Planktonic Bacteria

Powered by flagella, many bacterial species exhibit collective motion on a solid surface commonly known as swarming. Physical changes like cell elongation and hyper flagellation are known to accompany the swarming phenotype. However, less noticeable are the contrasts of collective motion between the swarming and planktonic cells of comparable density. Here, we show that when confined by microwells of specific sizes mounted on an agar surface, novel bacterium Enterobacter sp. SM3 under swarming condition exhibit a “single-swirl” motion pattern distinct from “multi-swirls” formed by its concentrated planktonic counterpart. We hypothesized that a “rafting behavior” of the swarming bacteria upon dilution might account for the motion pattern difference. We verified the conjecture via numerical simulations where swarming cells are modeled with lower repulsion and more substantial alignment. Our new technical approach also enabled us to observe swarming on a tissue surface and to perform physiologically relevant studies in the future.