Hydrodynamics and collective behavior of the tethered bacterium Thiovulum majus

Alexander Petroff

Center for Studies in Physics and Biology  

Rockefeller University

Over the course of the last several decades, the study of microbial communities has identified countless examples of interactions between microorganisms. To better understand one type of cooperation, how bacteria use hydrodynamics to manipulate their natural environment, we study the bacterium Thiovulum majus. T. majus cells attach to surfaces using a tether. As a tethered cell beats its flagella, it creates a flow that pulls nutrients to the cell. At high cell densities, the tethers become entangled to form a centimeter-scale cohesive matrix called a "veil". When attached to this matrix, cells pull nutrient-laden water through the veil an order of magnitude faster than could an individual. Here we investigate the hydrodynamics of cells and veils that underlies this remarkable form of cooperation. We begin by breifly discussing techniques for enriching these bacteria from the environment and maintaining a stable culture. We then measure the flow of water around a tethered cell and show that a tethered cell transports water much more efficiently than could a free-swimming cell. We then derive the equations of motion of tethered cells and show that as the density of cells passes a critical value, interactions between cells cause them to weave their tethers into a veil. Finally, we present observations of the macroscopic dynamics of a veil. We find that veils generate and move under the influence of centimeter-scale fluid flows. These results highlight how bacteria evolve strategies to exploit basic physical principles.