How Bacteria Swim and Tumble
A motile E. coli propels itself from place to place by rotating its flagella. To move forward, the flagella rotate counterclockwise and the organism "swims". But flagellar rotation may abruptly change to clockwise. When that happens, the bacterium just "tumbles" in place and seems incapable of going anywhere. Bacteria stay where there's lots of food and few dangerous
chemicals by tumbling more and swimming less. But as food is depleted,
tumbling give way to swimming, enabling the organisms to move to a more
hospitable environment.
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