Hydrodynamic simulations of bacterial swarms highlight the role of cell morphology and cell-cell interactions on emergent patterns

Swarming, a multicellular mode of flagella-based motility observed in many bacteria species, enables coordinated and rapid surface translocation, expansion and colonization. Swarming bacteria display collective features such as persistent aligned flocks, fluctuating velocity fields, and significant vortical structures; these striking patterns have all been observed previously in experiments on swarming bacteria. We explore the roles played by cell shape, direct cell-cell interactions and fluid-mediated hydrodynamic interactions in the onset and maintenance of these structures. Using agent-based simulations of synthetic bacteria modeled as self-propelling high aspect ratio dipolar rods, we explore the effects of geometry, bacterial activity and density on the emergence of flow patterns. High aspect ratios and reduced hydrodynamic interactions yield more persistent rafts and clusters. Conversely, increased hydrodynamic interactions yield reduced cluster sizes but high vorticity and pressure flow gradients. We find that hydrodynamic and steric interactions compete, each differently impacting the role of cell morphology. Our results motivate further studies elucidating how mixing, genetic drift, cell motility, and adaptation may be impacted by cell-cell interactions.