Fig. 2.

Rotation of individual gears and of gear assemblies. Sequences of snapshots illustrating rotation of gears with eight external teeth (A–D) and twelve internal teeth (E–H). Images (I) and (J) show a system of two “engaged” gears rotating in opposite directions. Black arrows indicate the gears’ orientation obtained by computer processing of acquired images, and red arrows show the direction of rotation. In all cases, concentration of bacteria was 2 × 10¹⁰ cm^-3 and the film thickness was 200 µm. Contrast of the images was adjusted electronically. Movies S1, S2, and S3 correspond to images A–J.