Scattering of a fast-swimming bacterium off of a surface

The sediment bacterium Thiovulum majus is one of the fastest known
bacteria. Each bacterium is spherical and covered in approximately 200
flagella, which propel cell at a speed of 600 micron/sec. When a cell
collides with a surface, it often becomes hydrodynamically trapped. A
bound cell exerts a force normal to the surface and rotates. These
cells self-organize into active two-dimensional crystals of rotating
cells. Here we present the first experimental observations of the
scattering of a fast-swimming bacterium off a flat surface. We show
that the probability that a cell becomes bound decays exponentially
with the angle at which the cell approaches the surface; cells
colliding head on with the surface are most likely to become
trapped. We examine the stability of a bound cell and measure the
timescale over which the cell orients normal to the surface. Finally
we show that when a cell escapes, it leaves the surface at a fixed
angle. These results give insight into the ecology of fast-swimming
sediment microbes as they navigate the water saturated pore spaces in
which they live.