Chemotaxis
Chemotaxis is a simple biological circuit that guides E. coli to nutrients, while avoiding toxins.
The feedback loop controls two outputs:
- How much to turn?
- How fast to go?
The feedback loop has two inputs:
- How much attractant is present? If a lot, go towards it.
- How much repellant is detected? If a lot, go away from it.
The feedback loop as a decision table is something like this:
| Direction | Velocity | |
|---|---|---|
| Steady-state | Random | Fixed |
| Attractant detected (e.g. nutrient) | Fixed(towards) | Faster |
| Repellant detected (e.g. toxin) | Fixed(away) | Faster |
The bacteria has receptors for different types of attractants and toxins. There is negative feedback on the turning motor speed for each type.
Sensory adaptation is achieved through methylation. Methylation markers make the attractant/toxin receptors more or less sensitive.
Source
An Introduction to Systems Biology: Design Principles of Biological Circuits by Uri Alon, Chapter 9, robustness in Bacterial Chemotaxis p153.