THE EFFECT OF PH ON THE GROWTH AND MOTILITY OF RHODOBACTER-SPHAEROIDES WS8 AND THE NATURE OF THE DRIVING-FORCE OF THE FLAGELLAR MOTOR

Rhodobacter sphaeroides WS8 grew, and swam vigorously, over the pH ran ge 6 to 9. Sustained motility was, however, observed in populations of cells resuspended at pH values between 4.9 and 10.4, although the mea n run speed was reduced at the extremes of pH. The ability of R. sphae roides to swim in strong alkaline conditions prompted the question of whether motility at alkaline pH was powered by a sodium motive force, as has been found in the facultative alkalophilic Bacillus and Vibrio species, particulary as motility was found to be sensitive to the sodi um channel inhibitor amiloride. The nature of the driving force of the flagellar motor was therefore investigated. It was found that R. spha eroides was motile over the same pH range in the absence and presence of sodium ions. The protonophore CCCP was found to inhibit motility un der all conditions, whereas monensin, an inhibitor of sodium pumps, ha d no effect upon motility in the presence or absence of sodium. It was concluded that the dp is the driving force for the flagellar motor in R. sphaeroides at all values of pH. Amiloride, a specific inhibitor o f the sodium-driven flagellar motor in alkalophilic Bacillus and Vibri o was shown to act non-specifically on the proton driven motor of R. s phaeroides, reducing the swimming speed of this organism in media with and without sodium to the same extent and over the complete pH range. Measurement of the Delta p by using the electrochromic absorbance cha nge of the carotenoid pigments to measure Delta Psi and P-31-NMR to me asure Delta pH showed that the maximum Delta p was about - 215 mV. At pH 10 the cells swam more slowly and the Delta p was about - 90 mV. Th ese data suggest that the flagellar motor of R. sphaeroides is proton- driven under all conditions with a threshold for motor rotation below - 90 mV and saturation at above - 90 mV and below - 215 mV.