Understanding how bacterial collectives organize on surfaces by tracking surfactant flow

•Swarming is a collective bacterial behavior on surfaces that can produce dendritic patterns, here referred to as tendrils. The mechanisms responsible for tendril development are not understood.•Bacterial sensing and fluid mechanics contribute to tendril development in the bacterium P. aeruginosa.•A novel technique, Imaging of Reflected Illuminated Structures (IRIS), reveals that tendril development is coupled with the growth of a surfactant layer.•Findings raise the possibility that the surfactant layer dynamics impacts tendril organization.