Bacteria swim by rotating the helical, hairlike flagella that extend from their unicellular bodies. Some bacteria, including the Escherichia coli (E. coli) living in the human gut, have multiple flagella that rotate as a bundle to move the cell forward. These cells turn somewhat acrobatically by unbundling their flagella, causing the cell to tumble, reorient and strike out in another direction.
But many microbes, including 90 percent of motile marine bacteria, have only a single rigid flagellum; they are able to swim both forward and backward by rotating this flagellum either counterclockwise or clockwise. These microbes change direction with a sideways "flick" of their lone flagellum -- a flick first documented in 2011 as a unique swimming stroke whose mechanism has remained a mystery.
Now, using high-speed video to record individual swimming bacteria at up to 1,000 frames per second, researchers at MIT have seen that the flick occurs when the so-called "hook," a small flexible rod connecting the flagellum to the cell's internal motor, buckles during forward swims. The drag on the cell head caused by the water's resistance combines with the opposing thrust force from the rotating flagellum to compress the hook, causing it to buckle and flick the cell into a 90-degree reorientation, the researchers say in a paper published online July 7 in Nature Physics. The ability to reorient by flicking ultimately helps bacteria make their way toward food in the nutrient-sparse ocean.
This might seem like an awkward means of navigation. But when you consider that marine bacteria can swim at nearly 100 body lengths per second the equivalent of a car traveling 900 mph, faster than the speed of sound -- and the flagellum spins at more than 1,000 revolutions per second, the 10-millisecond buckling of the hook seems a little more spectacular. The mechanism is of particular interest to engineers, who traditionally focus on the prev
|Contact: Denise Brehm|
Massachusetts Institute of Technology