Bacterial Chemotaxis.

http://en.wikipedia.org/wiki/Chemotaxis

Some bacteria, such as E. coli, have several flagella per cell (4–10 typically). These can rotate in two ways:

1. Counter-clockwise rotation aligns the flagella into a single rotating bundle, causing the bacterium to swim in a straight line.
2. Clockwise rotation breaks the flagella bundle apart such that each flagellum points in a different direction, causing the bacterium to tumble in place.

The directions of rotation are given for an observer outside the cell looking down the flagella toward the cell.