Kemotaksi

Kemotaksi, or chemotaxis, is the directed movement of an organism or cell in response to a chemical gradient. It is a fundamental mechanism by which cells locate nutrients, avoid toxins, and coordinate immune responses.

Organisms and cells that exhibit kemotaksi include bacteria such as Escherichia coli and Bacillus subtilis, single-celled organisms like Dictyostelium discoideum, and various mammalian cells such as neutrophils and dendritic cells. It also plays a role in developmental processes where cells migrate toward specific chemical signals.

Mechanism: In bacteria, chemotaxis relies on membrane-bound receptors called methyl-accepting chemotaxis proteins that detect attractants or repellents. These receptors regulate a two-component system involving the histidine kinase CheA and adaptor CheW, altering the phosphorylation state of CheY. Phosphorylated CheY modulates the rotation of the flagellar motor, biasing movement toward favorable conditions. Adaptation is achieved through CheR and CheB, allowing the system to reset after sustained stimuli.

In eukaryotes, chemotaxis commonly involves chemokine or growth factor receptors, typically G protein–coupled receptors or receptor tyrosine kinases. Activated receptors trigger signaling cascades, including the PI3K–PIP3 axis and Rho family GTPases, which polarize the cell and reorganize the actin cytoskeleton to form protrusions and direct movement toward higher concentrations of attractants or away from repellents.

Types: Positive chemotaxis toward attractants and negative chemotaxis away from repellents. Chemotaxis contributes to nutrient acquisition, immune cell targeting, wound healing, and developmental migration, while also playing roles in pathological contexts such as cancer metastasis.

Techniques: Experimental assays to study chemotaxis include capillary (Adler) assays, Boyden chambers, and microfluidic devices that create stable chemical gradients for quantitative analysis.