runandtumble

Run-and-tumble is a common locomotion pattern used by many flagellated bacteria. In this mode, cells alternate between relatively straight runs and brief reorientations called tumbles, allowing navigation through fluid environments.

Mechanistically, during a run the flagellar motors rotate in a way that produces a cohesive flagellar bundle, pushing the cell forward in a fairly straight trajectory. A tumble occurs when some motors switch to the opposite rotation, causing the bundle to disperse and the cell to reorient randomly. The switching is stochastic but regulated by the cell’s internal signaling network, which integrates environmental information to adjust the frequency and duration of runs and tumbles.

Chemotaxis, the movement toward favorable conditions and away from adverse ones, is closely tied to run-and-tumble. Bacteria sense chemical gradients with receptors that influence the motor bias via a signaling cascade. When moving toward attractants, the probability and length of runs increase, producing longer forward motions; when moving away, tumbles become more frequent, creating a biased random walk that tends to bring cells into favorable environments. Core components of the signaling system include sensor proteins, a histidine kinase, and response regulators that control motor rotation, along with adaptation mechanisms that fine-tune sensitivity.

Variations and occurrences vary by species. Run-and-tumble is well-characterized in peritrichously flagellated bacteria such as Escherichia coli and many Bacillus species, though related navigation strategies exist in other motile bacteria. In research contexts, run-and-tumble serves as a foundational model for studying microbial chemotaxis, motility patterns, and the design of synthetic biological systems.

See also: Chemotaxis, Bacterial motility, Flagellum.