Flagellum failure lets bacteria turn

When headed the wrong way, some bacteria turn by letting their propellers flop.

The newly discovered turning mechanism explains how a marine bacterium can control its direction using only a single flagellum, a stiff, rotating appendage that propels the cell forward. Turning depends on a mechanical characteristic that engineers might consider a failure if the flagellum were human-made: the tendency of flexible materials to buckle under pressure.

A multiflagellated bacterium like Escherichia coli turns by releasing one flagellum from a spinning bundle, which unwinds and sends the cell tumbling in a new direction. But 90 percent of mobile marine bacteria have only one flagellum each. In the past, scientists thought that these one-prop microbes could swim only in a straight line, says coauthor Roman Stocker of MIT. Then in 2011, a team led by Xiao-Lun Wu of the University of Pittsburgh showed that the single-flagellum bacterium Vibrio alginolyticus can make sharp turns. To change course, the cell backs up a little and swings its flagellum to one side, like a boat rudder.

But Wu’s team could not say how the bacterium controlled the flagellum flick. In research published July 7 in Nature Physics, Stocker’s team answered this question by filming V. alginolyticus with a high-speed video camera. His team found a crucial clue in the timing of the flick: It always happens about 10 milliseconds after the bacterium starts moving forward again.

The team guessed that the forward movement compresses the flagellar “hook,” which is the small flexible region that connects the flagellum to the cell. In reverse motion, the flagellum pulls on the cell; when the cell moves forward again the flagellum goes from pulling to pushing. The team showed that above a certain speed, the hook buckles, causing the flagellum to swing to one side.

But the researchers also found that the bacterium’s normal swimming speed was just as high as the speed that caused the flagellum to flop. “It should fail all the time,” Stocker says. “Whenever it tries to swim forward, it should just buckle.” Instead, they found that during steady forward swimming, the hook becomes much firmer. Stocker thinks the rotation of the flagellum may stiffen it.

The microbe’s minimalist approach to turning could work for tiny robots, says Bradley Nelson of the Institute of Robotics and Intelligent Systems at ETH Zurich. “It certainly provides inspiration to us to consider designing artificial flagella that also exhibit buckling.”