Response thresholds in bacterial chemotaxis

Stimulation of Escherichia coli by exponential ramps of chemoattractants generates step changes in the concentration of the response regulator, CheY-P. Because flagellar motors are ultrasensitive, this should change the fraction of time that motors spin clockwise, the CW(bias). However, early work failed to show changes in CW(bias) when ramps were shallow. This was explained by a model for motor remodeling that predicted plateaus in plots of CW(bias) versus [CheY-P]. We looked for these plateaus by examining distributions of CW(bias) in populations of cells with different mean [CheY-P]. We did not find such plateaus. Hence, we repeated the work on shallow ramps and found that motors did indeed respond. These responses were quantitatively described by combining motor remodeling with ultrasensitivity in a model that exhibited high sensitivities over a wide dynamic range.