Scattering of a fast-swimming bacterium off of a surface: Methods

Thiovulum majus is the fastest known bacteria. In its natural habitat, near the penetration depth of oxygen in salt marsh sediment, these bacteria create fluid flows that pull nutrient-rich water through the pore space. As cells navigate the pore space in which they live, they frequently collide with grains of sand. To better understand the ecology of these microbes, we investigate the dynamics of a collision of a T. majus with a solid surface. This presentation focuses on the experimental techniques needed to perform these measurements. These bacteria cannot be grown in pure culture. We first describe how we enrich T. majus from mud, collected from a Massachusetts salt marsh. Next, we analyze the motion of these cells in a microfluidic device. This device confines the motion of cells to two dimensions. We present techniques that allow us to track several thousand collisions between fast swimming cells and the chamber walls. We show that there are two types of collisions. If a cell approaches the wall at a glancing angle, it initially turns to swim parallel to the wall and then returns to the fluid. In the case of head-on collision, the cell becomes hydrodynamically bound to the surface.