• 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.