chemotaxis

Chemotaxis is a directed movement of cells or organisms in response to chemical gradients. Cells move toward higher concentrations of attractants or away from repellents. This behavior is observed in bacteria and diverse eukaryotes and plays a key role in foraging, immune defense, and development.

In bacteria, chemotaxis is mediated by chemoreceptors that sense chemicals and a signaling pathway that controls flagellar rotation. Receptors of the methyl-accepting chemotaxis protein (MCP) family modulate the activity of the CheA kinase via CheW. CheY, the response regulator, is phosphorylated and binds the flagellar motor to promote clockwise rotation and tumbles. When attractants bind, CheA activity decreases, reducing CheY phosphorylation and increasing smooth swimming. Adaptation to persistent stimuli occurs by reversible methylation of MCPs through CheR and CheB, restoring sensitivity to the gradient. The result is a biased random walk toward favorable conditions.

In many eukaryotes, chemotaxis relies on surface receptors such as G protein–coupled receptors or receptor tyrosine kinases that detect chemoattractants like chemokines, amino acids, or growth factors. Signaling cascades coordinate actin cytoskeleton rearrangements, leading to polarized cell protrusions and directional movement. Classic models include neutrophils in the immune system and the social amoeba Dictyostelium discoideum. Spatial sensing, temporal comparisons, and the local nature of gradients influence accuracy; some cells use gradient steepness and cell speed to optimize navigation.

Chemotaxis is studied in microbiology, immunology, and developmental biology, with applications in infection, wound healing, and cancer metastasis. Experimental tools such as microfluidic devices enable controlled gradient generation to observe migratory responses in real time.