Question: what is the external structure that allows a bacterium to run and tumble?
Answer: The peritrichous bacterium Escherichia coli is a well-studied example of a microswimmer in nature—one that is able to change its swimming direction actively, showing “run-and-tumble” motion. A cell swims by rotating multiple helical flagella, each of which is attached by a hook to a rotary motor on the body (1,2).