FoundationalConceptsNeuroscience

it then swims off again in a straight line for another second (a run).
Runs occur when the bacterium’s flagella rotate in one direction and
form a bundle that functions like a propeller that produces coherent
motion (Fig. 11.1). Tumbles occur when the direction of flagellar rota-
tion reverses, the individual flagella fly apart, forward motion ceases,
and the bacterial cell tumbles about randomly. The direction of flagel-
lar rotation then reverses again and another run commences, now in
a direction that is randomly related to that of the original run. These
bacteria thus swim in what is mathematically termed a random walk
in three dimensions.
An E. colicellis about 2 micrometers in length and swims along at
an impressive speed of about 30 micrometers per second, tumbling
off in a new direction every second or so. Now, suppose the bacterium
encounters a nutrient chemical substance. In particular, some amino
acids—such as aspartic acid and serine—interact with receptor
proteins located on the bacterial cell’s outer membrane and influ-
ence the bacterium to swim toward the nutrients—a behavior called
chemotaxis. Interaction of nutrients with the receptor proteins re-
duces the likelihood of tumbling and keeps the bacterium moving in
a direction where nutrients are present. If nutrient molecules become
less abundant, the bacterium reverts to its usual behavior of tumbling
every second or so and keeps changing direction until encountering
more nutrients, which then reduces the frequency of tumbles. The
bacterium continues to swim in a random walk, but its direction
is biased by swimming for longer time periods toward nutrients it
encounters.