Khemotaksis

Chemotaxis is directed movement of bacteria either towards a chemical attractant or away from a chemical repellent

The concentrations of these attractants and repellents are detected by chemoreceptors in the surfaces of the bacteria

Directional travel toward a chemoattractant (biased random walk toward attractant) is caused by lowering the frequency of tumbles (twiddles), thereby lengthening the runs when traveling up the gradient, but allowing tumbling to occur at normal frequency when traveling down the gradient

Directional travel away from a chemorepellent (biased random walk away from repellent) involves similar but opposite responses

The mechanism of control of tumbles and runs is complex, involving numerous proteins and several mechanisms (conformation changes, methylation, and phosphorylation) to modulate their activity; despite this complexity chemotaxis is fast, with responses occurring in as little as 200 meters/second