Bacteria swim by rotating the helical, hairlike flagella that extend from their unicellular bodies. Some bacteria 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 90 percent of motile marine bacteria have only a single rigid flagellum. These microbes change direction with a sideways “flick” of their flagellum — a unique swimming stroke first documented in 2011 whose mechanism has been a mystery. Now, using high-speed video to record individual swimming bacteria at up to 1,000 frames per second, Professor Roman Stocker, Kwangmin Son and Jeffrey Guasto show that the flick occurs when the so-called “hook,” a 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. Read a news release.