Now, the organism I study is E. coli, but the behavior I'm interested in is still orientation. How is a unicellular organism able to orient towards food and away from toxic substances?
The swimming behavior of E.coli (and other bacteria) is a "random walk" -- the bacterium swims in a straight line, but occasionally stops and heads off in a new random direction. These changes in direction are called tumbles. When the bacterium encounters some attractive substance (such as simple amino acids), it tumbles less frequently. As a result of this change in tumble frequency, E. coli will tend to follow a chemical gradient, i.e. they will tend to swim towards areas where there's more food. This behavior is known as chemotaxis.
Using a computerized motion-analysis system, we can track the swimming behavior of single bacteria over time. When the cells are exposed to an abrupt increase in attractant levels, we can measure a clear change in the cells' tumble frequency and speed of swimming.
In animals, the translation of sensory input to motor output (or stimulus to response) is mediated by the nervous system. In E.coli, this translation is mediated by a protein cascade. A great deal is known about these Che(motactic) proteins. A series of mutations have been made in the Che proteins, and these mutations result in predictable changes in the chemotactic behavior of the bacteria.
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