Most possess one or more flagella on their surface that allow them to swim. The pattern of flagellation is an important feature in identification of motile bacteria. The figure illustrates the commonly observed arrangements of flagella.
Polar flagella occur at one or both ends of the bacterium.
They may be single or in tufts. Peritrichous flagella are distributed around the surface of the organism. Most motile bacteria move in a straight line for a brief time, then turn and randomly change directions before swimming again. The straight line movement is called a run and the turn is called a tumble . Runs and tumbles are controlled by the clockwise or counterclockwise rotation of the basal body of the flagellum, the motor that is anchored in the cell membrane.