Transient response to chemotactic stimuli in Escherichia coli.

We have followed by eye and with the tracking microscope the rotational behavior of E. coli tethered to coverslips by their flagella. The cells change their directions of rotation at random, on the average about once a second. When an attractant is added or a repellent is subtracted, they spin clockwise (as viewed through the coverslip, i.e., along the flagellum toward the body) for many seconds, then counter-clockwise for many seconds, and then gradually resume their normal mode of behavior. The time interval between the onset of the stimulus and the clockwise to counter-clockwise transitiion is a linear function of the change in receptor occupancy. The cells adapt slowly at a constant rate to the addition of an attractant or the subtraction of a repellent. They adapt rapidly to the subtraction of an attractant or the addition of a repellent. Responses to mixed stimuli can be analyzed in terms of one equivalent stimulus.