The behavioural response of anaerobic Rhodobacter sphaeroides to temporal stimuli

The behavioural response of Rhodobacter sphaeroides to temporal changes in the concentration of chemoeffectors, and to stimuli affecting electron transport, was analysed using tethered cells. Populations of photosynthetically grown tethered cells of R. sphaeroides showed a transient response, a stop followed by adaptation, to a stepwise reduction in the concentration of chemoattractants (such as organic acids or sugars) and terminal electron acceptors. A step-down response was also measured in free swimming cells to a reduction in light intensity. As this response appears to apply to all effectors this suggests that there is a sensory pathway in anaerobically grown R. sphaeroides which responds primarily to a reduction in a stimulus. R. sphaeroides therefore responds when moving down a concentration gradient. This is the inverse of the classical Escherichia coli-Salmonella typhimurium model of bacterial sensory behaviour in which bacteria respond primarily when there is an increase in an attractant concentration, i.e. when moving up a gradient. R. sphaeroides does show a chemokinetic response to an increase in concentration of a limited number of compounds but this response is sustained and accompanied by an increase in the rate of flagellar rotation and therefore not simply equivalent to the transient increase in smooth swimming measured in E. coli on addition of an attractant.