THE BEHAVIORAL-RESPONSE OF ANAEROBIC RHODOBACTER-SPHAEROIDES TO TEMPORAL STIMULI

The behavioural response of Rhodobacter sphaeroides to temporal change s in the concentration of chemoeffectors, and to stimuli affecting ele ctron transport, was analysed using tethered cells. Populations of pho tosynthetically grown tethered cells of R. sphaeroides showed a transi ent response, a stop followed by adaptation, to a stepwise reduction i n the concentration of chemoattractants (such as organic acids or suga rs) and terminal electron accepters. A step-down response was also mea sured in free swimming cells to a reduction in light intensity. As thi s response appears to apply to all effecters this suggests that there is a sensory pathway in anaerobically grown R. sphaeroides which respo nds primarily to a reduction in a stimulus. R. sphaeroides therefore r esponds when moving down a concentration gradient. This is the inverse of the classical Escherichia coli-Salmonella typhimurium model of bac terial sensory behaviour in which bacteria respond primarily when ther e is an increase in an attractant concentration, i.e. when moving up a gradient. R. sphaeroides does show a chemokinetic response to an incr ease in concentration of a limited number of compounds but this respon se is sustained and accompanied by an increase in the rate of flagella r rotation and therefore not simply equivalent to the transient increa se in smooth swimming measured in E. coli on addition of an attractant .