The sensitivity, or gain, of the chemotactic response is usually defined as the ratio of the (peak) change in motor bias to the change in receptor occupancy when a step change in the concentration of a chemoeffector is made. The maximum theoretical gain of the chemotaxis system can be estimated by assuming that an increase of 1% in the occupancy of receptors by aspartate results in an equal decrease in CheYp levels of 1%. Given that the wild-type bias is 0.85 and the degree of cooperativity between CheYp and the motor is 10.3, a simple calculation produces a value for the gain of 1.2, i.e. a barely enhanced sensitivity, whereas experimental observations have shown the system to be highly sensitive with gains from 6 to 60 reported.

 

Sensitivity of the standard chemotaxis model

The current model of the chemotaxis pathway fails to come close to matching the theoretical gain, with BCT 4.3 producing a value of 0.16 (although this rises to 0.9 if the Kds for binding to methylated and unmethylated receptors are made the same). This lack of sensitivity means that significant responses to aspartate only occur with step-changes (from a zero aspartate background) greater than 0.1 µM:

 

Enhancing the sensitivity of the model

As set out in Conformational spread and implemented in BCT, one way of enhancing the sensitivity of the response is to make the change in activity caused by the binding of a chemoeffector to one receptor spread to an arbitrary number of its neighbours. Using this mechanism, the simulated response to a sub-threshold step-change of 1nM aspartate becomes progressively larger as the number of recruited neighbours is increased (resulting in gains of 0.16, 9.0, 90 and 900, respectively):