Bacterial Swarming Motility Is both Confined and Facilitated by an Interfacial Fluid Film

Swarming is a prevalent form of bacterial collective motility. Our ongoing studies focus on two species of swarming bacteria, Pseudomonas aeruginosa and Enterobacter sp. SM3. In one project, we probe the swarm front on agar, a semi-solid substrate that provides nutrients for bacterial growth. By depositing a fluid droplet at the swarm edge, we observe a monolayer of swarming bacteria and track their motion. To study the effects of this thin layer confinement on swarming behavior, we analyze the velocities, orientations, and mean square displacements of bacteria within and outside the dynamic packs they form, as well as the motion of tracer beads to detect local fluid flow. We find that the air-liquid and liquid-solid interfacial confinements profoundly influence the swarming behavior. In a second project, we incorporate mucin, the major component of mucus found on airway and intestinal surfaces, into an agar gel to study its effects on the swarming motility. We find that mucin promotes bacterial swarming by inhibiting contact line pinning and making the agar surface more slippery. In a broad perspective, understanding the physical mechanisms that account for bacterial swarming motility may prove useful in biomedical and environmental applications.