SIMULTANEOUS MEASUREMENT OF BACTERIAL FLAGELLAR ROTATION RATE AND SWIMMING SPEED

Swimming speeds and flagellar rotation rates of individual free-swimmi ng Vibrio alginolyticus cells were measured simultaneously by laser da rk-field microscopy at 25, 30, and 35 degrees C. A roughly linear rela tion between swimming speed and flagellar rotation rate was observed. The ratio of swimming speed to flagellar rotation rate was 0.113 mu m, which indicated that a cell progressed by 7% of pitch of flagellar he lix during one flagellar rotation. At each temperature, however, swimm ing speed had a tendency to saturate at high flagellar rotation rate. That is, the cell with a faster-rotating flagellum did not always swim faster. To analyze the bacterial motion, we proposed a model in which the torque characteristics of the flagellar motor were considered. Th e model could be analytically solved, and it qualitatively explained t he experimental results. The discrepancy between the experimental and the calculated ratios of swimming speed to flagellar rotation rate was about 20%. The apparent saturation in swimming speed was considered t o be caused by shorter flagella that rotated faster but produced less propelling force.