Fig. 3

Single-cell motion pattern of P. mirabilis near diluted colony edge. a A sequence of phase contrast images recording the CW turning of an isolated cell colliding with the diluted colony edge (“Methods”). The dark area in upper portion of images is virgin agar. Scale bar, 5 μm. b Trajectory of the cell in panel a generated by overlaying a series of phase contrast images. Magenta arrows by the side of trajectory (white dotted line) indicate instantaneous moving direction of the cell. See Supplementary Movie 8. θ denotes interaction angle, defined as the angle between the diluted colony edge and the cell orientation just before collision. Scale bar, 20 μm. c Representative trajectories (N = 31) of cells that collided with diluted colony edge. Following the coordinate system specified in panel b, X = 0 μm is set at the edge of liquid drops. The starting point of all trajectories are aligned and set as Y = 0 μm and X = 55 μm. Blue (or magenta) lines represent the trajectory of cells that turned CW (or CCW) around the contact point during collision. d Tangential position of cells (whose trajectories were shown in panel e) plotted against time. The collision positions of all trajectories are aligned and set as Y = 0 μm, and T = 0 is set at the time of collision. Positive slope at T > 0 indicates that the cell moved in CW sense around the colony after collision. e Probability distribution of cells’ motion bias after collision with diluted colony edge plotted against interaction angle (Blue column: CW bias; Magenta column: CCW bias). f Model of a swimming bacterium in contact with liquid drop edge (top view). The torque driving reorentation G_drive and the drag torque G_drag balance, which leads to rotation at a constant angular speed ω about the x = 0; y = 0 point (green dot). The flagellar bundle is rotating at an angular speed Ω. The length of cell body and flagellar bundle is L_b and L_f, respectively. Also see “Methods”