Confinement Discerns Swarmers from Planktonic Bacteria

Powered by flagella, many bacterial species exhibit collective motion on a solid surface commonly known as swarming. As a natural example of active matter, swarming is also an essential biological phenotype associated with virulence, chemotaxis, and host pathogenesis. Physical changes like cell elongation and hyper flagellation have been shown to accompany the swarming phenotype. However, less noticeable, are the contrasts of collective motion between the swarming cells and the planktonic cells of comparable cell density. Here, we show that confining bacterial movement in designed dimensions allows distinguishing bacterial swarming from collective swimming. We found that on a soft agar plate, a novel bacterial strainEnterobactersp. SM3 exhibited different motion patterns in swarming and planktonic states when confined to circular microwells of a specific range of sizes. When the confinement diameter was between 40 μm and 90 μm, swarming SM3 formed a single swirl motion pattern in the microwells whereas planktonic SM3 showed multiple swirls. Similar differential behavior is observed across a range of randomly selected gram-negative bacteria. We hypothesize that the “rafting behavior” of the swarming bacteria upon dilution might account for the motion pattern difference. We verified our conjectures via numerical simulations where swarming cells are modeled with lower repulsion and more substantial alignment force. The novel technical approach enabled us to observe swarming on a non-agar tissue surface for the first time. Our work provides the basis for characterizing bacterial swarming under more sophisticated environments, such as polymicrobial swarmer detection, andin vivoswarming exploration.