Fig 1. A sketch of the motor response curve and the dependence of the up-gradient chemotactic velocity on the running time.
The curve represented in the main panel is a cartoon version of the clockwise (CW) bias vs the concentration of the second messenger CheYp of the chemotaxis pathway. Having the system set at the inflection point of the curve (blue point) would maximize the slope, i.e. the absolute sensitivity of the motor. However, the bacterial up-gradient velocity is not simply proportional to the absolute sensitivity, as discussed in the text. In particular, extending the duration of the runs can speed up the velocity as shown in the inset: the running time is reported in seconds on the abscissae while the solid line is Eq (4) with λ = 1 and the dashed line is again Eq (4) but λ changes now with τr so as to maximize the chemotactic velocity. The upshot is that points on the motor curve which do not maximize the absolute sensitivity, like the red one, can actually yield larger up-gradient speeds.