Microscopic 'clutch' puts flagellum ...(s are mobile or stationary. Although B. ...)

Once the scientists learned EpsE was involved in repressing flagellar motion, they devised two possible explanations for how EpsE acts. The first was that EpsE acts like a brake by pushing a non-moving part against a moving part and locking up the works. The other possibility, they imagined, was that EpsE acts like a clutch, disengaging one moving part from another. In this latter scenario, the engine can no longer drive flagellar spinning because key moving parts are no longer in contact. In this case, the flagellum would still have freedom of motion, listless as it might be.

To determine which hypothesis was correct, the scientists decided it best to let the tail wag the dog. They attached the tail end of the flagellum to a glass slide and examined the movement of the entire cell in the presence and absence of EpsE. In the absence of EpsE, the entire cell rotated once every five seconds. In the presence of EpsE, the cells stopped but could rotate passively, pushed by disturbances in the environment (Brownian motion). If EpsE acted like a brake, the cells would not have rotated at all.

The researchers also learned that when the cell begins producing EpsE, it takes about 15 minutes before the flagellar machinery is disabled.

"This makes a lot of sense as far as the cell is concerned," Kearns said. "The flagellum is a giant, very expensive structure. Often when a cell no longer needs something, it might destroy it and recycle the parts. But here, because the flagellum is so big and complex, doing that is not very cost effective. We think the clutch prevents the flagellum from rotating when constrained by the sticky matrix of the biofilm."

The discovery may give nanotechnologists ideas about how
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