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Bundling

Bundling is an event that can happen in multi-flagellated cells, bundling the flagella together and causing them to rotate in a coordinated manner.

Flagella are left-handed helices, and when rotated counter-clockwise by their rotors, they can bundle and rotate together. When the rotors reverse direction, thus rotating clockwise, the flagellum unwinds from the bundle. This may cause the cell to stop its forward motion and instead start twitching in place, referred to as tumbling. Tumbling results in a stochastic reorientation of the cell, causing it to change the direction of its forward swimming.

It is not known which stimuli drive the switch between bundling and tumbling, but the motor is highly adaptive to different signals. In the model describing chemotaxis ("movement on purpose") the clockwise rotation of a flagellum is suppressed by chemical compounds favorable to the cell (e.g. food). When moving in a favorable direction, the concentration of such chemical attractants increases and therefore tumbles are continually suppressed, allowing forward motion; likewise, when the cell's direction of motion is unfavorable (e.g., away from a chemical attractant), tumbles are no longer suppressed and occur much more often, with the chance that the cell will be thus reoriented in the correct direction.

Even if all flagella would rotate clockwise, however, they often cannot form a bundle due to geometrical and hydrodynamic reasons.