The effect of phosphate on the motility of Rhodobacter sphaeroides

When Rhodobacter sphaeroides was resuspended in sodium phosphate buffer directly after growth on complete medium there was an almost complete loss of motility. The addition of 1 mM sodium phosphate to R. sphaeroides suspended in Hepes buffer also caused an immediate reduction in the mean swimming speed of 35%; an effect that was maintained for more than 60 rain. This loss of motility was prevented by the inclusion of succinate in the resuspension medium. Addition of phosphate in the absence of succinate also increased the membrane potential by up to 35% and decreased the rate of respiration by 25%. These effects were reversed in the presence of succinate, phosphate addition decreased the membrane potential by up to 7% and increased the rate of respiration by 23%. These data show that the effect of phosphate buffer on R. sphaeroides is highly complex: and its inhibition of motility is probably an indirect effect. The use of phosphate buffer in many physiological experiments may have unexpected side effects.