Bacterial chemotaxis signal response: allows bacteria to navigate. Chemicals are either attractants or repellants.
Common attractants: sugars and amino acids serene and aspartate, and repellents such as certain metal ions and amino acid leucine.
bacteria can detect concentration gradients as small as a change of one molecule per cell volume across the cell length. They detect such gradients over background concentrations spanning five orders of magnitude. (no reference)
Biased random walk: walk straight for 1sec and then tumbes in about 0.1 sec. Ecoli keeps track of the gradient and then adjusts the tumbling frequency accordingly.
Chemotaxis sensses the temporal derivative of the concentration of attractants and repellents.
Animal cells, whose cell size is about $10\mu m$ and whose response takes minutes, can sense spatial gradients directly.
An experiment: Add Aspartate to a liquid sample. The steady state tumbling frequency increases in a unifrom distribution of the attractant. As the cell senses an “increasing gradient” in all directions. The frequency reduces from $1Hz$ to $0.1Hz$. Eventually, it settles back to the steady state value, in a response termed as sensory adaptation. This takes several seconds to several minutes based on the step change.
Bacterial chemotaxis shows exact adaptation, that is the steady-state tumbling frequency is well defined and would fallback to the same value with every step increase of the attractant. That is, the steady-state tumbling frequencyt is independent of the attractant levels. Barrying that the attractant concentration does not saturate the receptors.
The tumbling frequency is mediated by the probablity of the flagella motors to turn Clockwise (CW), heras the usual state is CCW and in synchrony.
Robustness in Bacterial Chemotaxis
Notes from Uri Alon's book (Chapter 9)