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Chemotaxis is directed movement of biological cells in response to a chemical concentration gradient. A chemoattractant biases movement toward higher concentration; other signals can bias movement away. At the population scale, chemotactic drift contributes a chemotactic flux that can compete with diffusion and fluid flow.

The directional bias need not mean that each cell points continuously toward the source. In swimming bacteria, changes in the durations of relatively straight movement and reorientation can bias an otherwise wandering trajectory. A motivating experiment changes the spatially uniform concentration of attractant rapidly while observing cell motion. Increased and decreased stimulation produce different changes in movement, demonstrating a temporal response without requiring a front-to-back gradient across the cell.^[ref] This temporal sensing in bacterial chemotaxis explains how successive samples during movement can guide migration through a spatial chemical landscape.

Researcher in biochemical signaling. With Robert M. Macnab, Koshland investigated gradient sensing in bacterial chemotaxis, connecting changes in an external chemical stimulus with control of movement.^[ref]

For the same concentration gradient, the front-to-back difference scales with cell length. A much smaller bacterial cell samples a correspondingly smaller spatial signal, while stochastic arrival and binding of individual chemoattractant molecules make receptor measurements noisy. For illustration, a $2\mu \mathrm{m}$ cell in the $0.5$ cm capillary would experience only $0.04\mathrm{nM}$ across its length, compared with $0.4\mathrm{nM}$ for the longer cell. This illustrates a size penalty; it does not assume that bacterial and human detection thresholds are identical.

Many swimming bacteria, including Escherichia coli, use temporal sensing in bacterial chemotaxis: they compare recent receptor stimulation with an adapted baseline as they move. Increasing attractant suppresses reorientation and prolongs runs; decreasing attractant increases tumbling. Repeated runs and reorientations therefore generate a biased random walk up the gradient, without requiring a simultaneous comparison between the front and back. Chemotaxis feedback regulation supplies the adaptation needed to remain sensitive to changes rather than responding only to absolute concentration.

A discriminating experiment changes concentration rapidly and uniformly around the bacteria: a response then occurs without a spatial difference across the cell. The classic temporal-step experiments found increased tumbling after attractant decreases and smoother swimming after increases.^[ref] In a new experiment, compare up-steps and down-steps with constant-concentration and sham-mixing controls, holding temperature and medium composition fixed. Measure run durations and tumble frequency, rather than only average swimming speed. Such a test supports temporal comparison but does not by itself identify every molecular component or show that all bacterial species use the same mechanism.

Researcher in bacterial motility. With Daniel E. Koshland Jr., Macnab studied gradient sensing in chemotaxis, helping explain how a moving bacterium can extract directional information from changes in its environment.^[ref]

Temporal sensing in bacterial chemotaxis compares changing stimulation during movement with a recent adapted baseline. Attractant increases can lengthen runs and decreases can increase tumbling, producing biased movement up a chemical gradient. Rapid spatially uniform concentration steps distinguish a temporal response from a requirement for front-to-back spatial comparison.^[ref]