Bacterial Chemotaxis

E. coli run-and-tumble navigation up a chemoattractant gradient

Run duration λ: 2.0 s Gradient ∇C: 0.50 Bacteria N: 15

E. coli swims in straight "runs" (CCW flagella bundle) punctuated by "tumbles" (CW rotation, random reorientation). Chemotaxis arises from adaptation: if attractant concentration C increases during a run, the tumble rate λ decreases (CheY-P suppressed), extending runs up-gradient. The drift velocity v_d = v_run × (λ⁻ - λ⁺)/(λ⁻ + λ⁺) is proportional to ∇C, producing the biased random walk.