Run-and-tumble motion

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Run-and-tumble motion is a simple way some bacteria and tiny organisms move. It alternates between runs and tumbles. During a run, the cell moves in a roughly straight line in a fixed direction. During a tumble, it mostly stops and reorients, preparing for the next run. The changes in direction are random, like a hop from one direction to another, and the length of each run is also random.

For many bacteria, such as E. coli in a dilute liquid, runs last about a second on average, while tumbles are much shorter, around a tenth of a second. The overall path looks like a series of straight, nearly parallel stretches (the runs) with quick, random turns (the tumbles) in between.

How the motion is controlled. Bacteria swim with flagella, tiny tails that rotate like tiny propellers. When the flagella rotate in one direction, they form a bundle that pushes the cell forward (a run). When they switch and rotate in the other direction, the bundle falls apart and the cell tumbles, changing direction. The direction of rotation is guided by chemical signals detected by receptors on the cell surface. If the cell senses an attractant (like a nutrient), it tends to keep running longer in a good direction. If it senses a repellent, it tumbles more and changes direction more often.

Different organisms use this strategy in different ways. Many bacteria, such as E. coli, Salmonella, and Bacillus subtilis, show classic run-and-tumble swimming. Some algae, like Chlamydomonas, and cyanobacteria, such as Synechocystis, also show related intermittent movements. Synechocystis, which doesn’t have flagella, can move using pili and shows two-phase motion: a high-motility run and a lower-motility tumble. Light can affect its motion: uniform bright light makes runs more likely in all directions, while strong light or ultraviolet radiation can switch behavior or even stop movement to protect the cell.

Why this matters. Run-and-tumble motion helps these organisms explore their surroundings and find good places to live or grow. It’s a simple, effective way to navigate complex environments using random motion biased by cues like chemicals or light.

In science, this pattern is also used to build simple models of how self-propelled particles move. These models help researchers study movement in crowded or complicated settings and relate biological motion to other areas of physics.