Kemotaxis

Kemotaxis (chemotaxis) is the movement of an organism or cell in response to a chemical concentration gradient. It can be toward higher concentrations of attractants or away from repellents, and it is a key mechanism in development, immune response, and microbial ecology.

In bacteria such as Escherichia coli, chemotaxis uses a biased random walk. Chemo-receptors detect attractants or repellents and relay signals to a histidine kinase CheA and the response regulator CheY via adaptor proteins. Attractants lower CheY phosphorylation, reducing tumbles and increasing straight runs; repellents raise CheY-P, increasing turning. Adaptation occurs by receptor methylation and demethylation through CheR and CheB, resetting sensitivity.

In eukaryotes, chemotaxis requires coordinated signaling that polarizes the cell and reorganizes the cytoskeleton. Chemoattractants binding to G protein–coupled receptors can activate PI3K, generating PIP3 and directing actin assembly at the leading edge. Neutrophils, lymphocytes, and Dictyostelium discoideum migrate toward gradients of amino acids, chemokines, or cAMP, using spatial sensing and adhesion cues to move directionally.

Chemotaxis is distinct from chemokinesis, which changes the rate of movement without directional bias, and from taxis in a broader sense, which describes directional movement in response to stimuli other than chemicals such as light (phototaxis) or oxygen (aerotaxis).

Its study informs understanding of host defense, cancer metastasis, wound healing, and environmental microbiology, and underlies applications in drug development and bioremediation.