A bacterium swims like a tiny motorboat with a propeller — except its propeller is a long flagellum that rotates and pushes the body forward. But the body doesn't move in a straight line; it sways like a bobber on a wave. This swaying isn't random: it's part of the mechanics of motion.
Until now, such swaying was studied in still water. But what happens when the water flows? In nature, bacteria almost always live in moving fluids: in rivers, in the gut, in pipes. Scientists placed E. coli in a flow and filmed it with a camera.
The reason turned out to be a flexible "hook"—a tiny part connecting the bacterium's body to its flagellum. When the flow pushes on the body, the hook bends, and the flagellum starts pulling at an angle. The body sways more, but the speed drops: some energy goes into wiggling rather than moving forward.
This discovery helps us understand how bacteria spread in nature and in the body — for example, in the gut or soil, where there is always a fluid current. Now it's clear: the stronger the flow, the worse the bacterium advances against the current, but the more actively it "veers" sideways.
The flagellum of E. coli rotates at up to 100 revolutions per second — faster than a race car engine.