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.

Source: Buckling up to turn: Marine microbe... via a high-speed mechanical instability