FIG. 5.

Typical swarm-cell reversal. A cell labeled with Alexa Fluor 488 is shown to change its direction by backing up. (A) The cell moves downwards and starts the reversal process. (B) The flagellar motors have reversed, and the bright green dots are the filament transformation points between normal (left-handed) and curly (right-handed) forms. The cell body has changed its direction of motion. (C) The loosened bundle appears folded, and the cell body has moved through the center of the loosened bundle to extend past the filaments’ distal tips. Thus, the cell has backed up without changing its orientation. (D) The bundle has reformed with curly filaments, and the cell now swims upwards. This maneuver is depicted schematically in the right panel. Eventually, the flagellar motors switch back to CCW operation, and the filaments transform to normal (not shown). The cell body is highlighted in gray to aid the eye, and the phase-contrast images of other cells in the background appear dark. The frames are 1/10 s apart.