Tumble / Run

Escherichia coli execute a random walk by alternating between two modes of swimming, running and tumbling. They swim by means of rotating helical flagella. When the flagella rotate counterclockwise, the bacterium swims in a smooth, directed manner. When the flagella rotate clockwise, the bacterium tumbles. Tumbles serve to reorient the bacterium so it will swim in a different (randomly-chosen) direction. When the bacterium swims up gradients of chemical attractants, tumbles are less frequent and the runs thus become longer. When swimming down steep gradients, the bacterium tumbles more frequently and runs are shorter.

In other words: E. coli moves around more or less at random, letting the mechanics of the environment and the whims of its flagella take it where they will. In environments it likes it tries to move less randomly; in environments it dislikes it moves more randomly. In any case a lot about how it decides its path through life is just fundamentally random, although the net result is very much determined by the environment.