Dynamics in a Monolayer of Bacteria at the Swarm Front

Swarming behavior is a trait observed in many species of flagellated bacteria. It occurs when a growing population of motile cells expands on an agar surface, resulting in collective motion. We investigate the swarming behavior of Enterobacter sp. SM3, a gut bacterium that has recently been identified and shown to provide a physiological benefit to lab mice suffering intestinal inflammation. In our experiments, SM3 cells at the swarm front organize into dynamic packs within a monolayer of cells, where strong but transient cell-cell interactions are essential to the swarm’s collective behavior. We closely track individual cells’ movements and trajectories within these packs. We develop a force dipole model using two beads to model each bacterium with explicit consideration of cell density and aspect ratio, in order to predict critical steric and hydrodynamic interactions that drive swarm dynamics. The aim of our combined experimental and computer simulation study is to determine the forces underlying bacterial swarming, with a particular emphasis on how hydrodynamics influence the coordinated movement within the swarm.