Although bacteria are unicellular and among the simplest autonomous forms of life, they show a remarkable ability to sense their environment. They are attracted to materials they need and are repelled by harmful substances. Most types of bacteria swim very erratically; short smooth runs in relatively straight lines are followed by brief pattern consists only of smooth runs and tumbles, the latter resulting in random changes in direction.
One clue comes from the observation that when a chemical attractant is added to a suspension of such bacteria, the bacteria swim along a gradient of the attractant, from an area where the concentration of the attractant is weaker to an area where it is stronger. As they do so, their swimming from it are shortened by an increased tendency of the bacteria to tumble and change direction.
Biologists have proposed two mechanisms that bacteria might use in detecting changes in the concentration of a chemical attractant. First, a bacterium might compare the concentration of a chemical at the front and back of its cell body simultaneously. If the concentration is higher at the front of the cell, then it cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times.
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