Coliform bacteria slowly adapt to the presence
of attractants and repellents by altering the methylation state
of their receptors to restore the pre-stimulus swimming behaviour.
These methylation and demethylation reactions can be implemented
in either a fine-tuned or a robust manner [Barkai & Leibler
(1997) Nature 387:913-917]. In the fine-tuned approach, the net
rate of receptor modification depends on the concentrations
of the receptor species; in the robust approach, the net rate
of receptor modification depends on the activities of the
receptor species. With a square pulse of aspartate from 100 s
to 600 s for a range of aspartate concentrations, the differences
between the two approaches become apparent.
Fine-tuned adaptation to aspartate (BCT 4.0)
Here, the reaction network has been fine tuned to give exact
adaptation at 1 µM aspartate. At any other concentration,
adaptation still occurs but is no longer exact.
Robust (exact) adaptation to aspartate (BCT 4.2)
Note that de-adaptation is more rapid than adaptation and that
adaptation takes longer at high aspartate levels. Both these features
of robust adaptation are in accord with experimental observations.
Robust (exact) adaptation to nickel (Ni++)
