  We examined the changes in swimming behaviour of the bacterium Rhodobacter
sphaeroides in response to stepwise changes in a nutrient (propionate),
following the prestimulus motion, the initial response and the adaptation to
the sustained concentration of the chemical. This was carried out by tethering
motile cells by their flagella to glass slides and following the rotational
behaviour of their cell bodies in response to the nutrient change. Computerised
motion analysis was used to analyse the behaviour. Distributions of run and
stop times were obtained from rotation data for tethered cells. Exponential and
Weibull fits for these distributions, and variability in individual responses
are discussed. In terms of parameters derived from the run and stop time
distributions, we compare the responses to stepwise changes in the nutrient
concentration and the long-term behaviour of 84 cells under twelve propionate
concentration levels from 1 nM to 25 mM. We discuss traditional assumptions for
the random walk approximation to bacterial swimming and compare them with the
observed R. sphaeroides motile behaviour.
