The Organized Melee: Emergence of Collective Behavior in Concentrated Suspensions of Swimming Bacteria and Associated Phenomena

Abstract

Suspensions of the aerobic bacteria {\it Bacilus subtilis} develop patterns and flows from the interplay of motility, chemotaxis and buoyancy.In sessile drops, such bioconvectively driven flows carry plumes down the slanted meniscus and concentrate cells at the drop edge, while in pendant drops such self-concentration occurs at the bottom.These dynamics are explained quantitatively by a mathematical model consisting of oxygen diffusion and consumption, chemotaxis, and viscous fluid dynamics.Concentrated regions in both geometries comprise nearly close-packed populations, forming the collective ``Zooming BioNematic'' (ZBN) phase.This state exhibits large-scale orientational coherence, analogous to the molecular alignment of nematic liquid crystals, coupled with remarkable spatial and temporal correlations of velocity and vorticity, as measured by both novel and standard applications of particle imaging velocimetry.To probe mechanisms leading to this phase, response of individual cells to steric stress was explored, finding that they can reverse swimming direction at spatial constrictions without turning the cell body.The consequences of this propensity to flip the flagella are quantified, showing that "forwards" and "backwards" motion are dynamically and morphologically indistinguishable.Finally, experiments and mathematical modeling show that complex flows driven by previously unknown bipolar flagellar arrangements are induced when {\it B. subtilis} are confined in a thin layer of fluid, between asymmetric boundaries.The resulting driven flow circulates around the cell body ranging over several cell diameters, in contrast to the more localized flows surrounding free swimmers.This discovery extends our knowledge of the dynamic geometry of bacteria and their flagella, and reveals new mechanisms for motility-associated molecular transport and inter-cellular communication.