  Bacterial processes ranging from gene expression to motility and biofilm
formation are constantly challenged by internal and external noise. While the
importance of stochastic fluctuations has been appreciated for chemotaxis, it
is currently believed that deterministic long-range fluid dynamical effects
govern cell-cell and cell-surface scattering - the elementary events that lead
to swarming and collective swimming in active suspensions and to the formation
of biofilms. Here, we report the first direct measurements of the bacterial
flow field generated by individual swimming Escherichia coli both far from and
near to a solid surface. These experiments allowed us to examine the relative
importance of fluid dynamics and rotational diffusion for bacteria. For
cell-cell interactions it is shown that thermal and intrinsic stochasticity
drown the effects of long-range fluid dynamics, implying that physical
interactions between bacteria are determined by steric collisions and
near-field lubrication forces. This dominance of short-range forces closely
links collective motion in bacterial suspensions to self-organization in driven
granular systems, assemblages of biofilaments, and animal flocks. For the
scattering of bacteria with surfaces, long-range fluid dynamical interactions
are also shown to be negligible before collisions; however, once the bacterium
swims along the surface within a few microns after an aligning collision,
hydrodynamic effects can contribute to the experimentally observed, long
residence times. As these results are based on purely mechanical properties,
they apply to a wide range of microorganisms.
