  Bacterial motility, and in particular repulsion or attraction towards
specific chemicals, has been a subject of investigation for over 100 years,
resulting in detailed understanding of bacterial chemotaxis and the
corresponding sensory network in many bacterial species. For Escherichia coli
most of the current understanding comes from the experiments with low levels of
chemotactically-active ligands. However, chemotactically-inactive chemical
species at concentrations found in the human gastrointestinal tract produce
significant changes in E. coli's osmotic pressure, and have been shown to lead
to taxis. To understand how these nonspecific physical signals influence
motility, we look at the response of individual bacterial flagellar motors
under step-wise changes in external osmolarity. We combine these measurements
with a population swimming assay under the same conditions. Unlike for
chemotactic response, a long-term increase in swimming/motor speeds is
observed, and in the motor rotational bias, both of which scale with the
osmotic shock magnitude. We discuss how the speed changes we observe can lead
to steady state bacterial accumulation.
