Swimming speed and chemokinetic response of Rhodobacter sphaeroides investigated by natural manipulation of the membrane potential.

The delta psi of R. sphaeroides, grown under high light to reduce the levels of light-harvesting bacteriochlorophyll, was naturally manipulated using light intensity. The relationship between delta psi and the swimming speed of free swimming populations of cells was investigated. After de-energisation by incubation in the dark there was an apparent threshold of about -13 mV which had to be overcome before functional motor rotation could resume and at -45 mV the motor saturated. Further increases in delta psi over -45 mV did not increase the free swimming velocity. However, when a chemokinetic effector was added there was an increase in swimming speed, even though the delta psi was well above saturation, indicating that the chemokinetic response is independent of normal relationship between motor rotation and delta psi.