Transformations in flagellar structure of Rhodobacter sphaeroides and possible relationship to changes in swimming speed

Rhodobacter sphaeroides is a photosynthetic bacterium which swims by rotati ng a single flagellum in one direction, periodically stopping, and reorient ing during these stops, Free-swimming R, sphaeroides was examined by both d ifferential interference contrast (DIC) microscopy, which allows the flagel la of swimming cells to be seen in vivo, and tracking microscopy, which tra cks swimming patterns in three dimensions. DIC microscopy showed that when rotation stopped, the helical flagellum relaxed into a high-amplitude, shor t-wavelength coiled form, confirming previous observations. However, DIC mi croscopy also revealed that the coiled filament could rotate slowly, reorie nting the cell before a transition back to the functional helix. The time t aken to reform a functional helix depended on the rate of rotation of the h elix and the length of the filament. In addition to these coiled and helica l forms, a third conformation was observed: a rapidly rotating, apparently straight form, This form took shape from the cell body out and was seen to form directly from flagella that were initially in either the coiled or the helical conformation. This form was always significantly longer than the c oiled or helical form from which it was derived. The resolution of DIC micr oscopy made it impossible to identify whether this form was genuinely in a straight conformation or was a low-amplitude, long-wavelength helix. Examin ation of the three-dimensional swimming pattern showed that R, sphaeroides changed speed while swimming, sometimes doubling the swimming speed between stops. The rate of acceleration out of stops was also variable. The transf ormations in waveform are assumed to be torsionally driven and may be relat ed to the changes in speed measured in free-swimming cells. The roles of an d mechanisms that may be involved in the transformations of filament confor mations and changes in swimming speed are discussed.