Fig. 1: Schematic of multiscale chemotaxis assay and its application to E. coli strain AW405.

a A quasi-static linear chemical gradient is established between two reservoirs containing a uniform concentration of bacteria. Bacteria are observed in the central portion of the linear gradient starting 50 min after filling the reservoirs. b 5045 individual trajectories with a minimum duration of five frames and containing 37,080 s of total trajectory time, obtained in 9 min of recording at 15 Hz in a typical experiment. c Two example trajectories (durations 63 and 65 s) showing run-tumble motility in bulk solution and circular segments near the chamber surface (within 10 µm distance, faded). d Drift velocity (black, defined as the average speed along the gradient direction, x) and average swimming speed (gray) as a function of height, z, computed from three biological replicates of the experiment, comprising 9903 motile trajectories with a combined duration of 79,562 s. Only bulk trajectories (defined as trajectory segments with a distance of more than 10 µm to the surface) are retained for further analysis. Error bars reflect standard errors of the mean. e Bulk trajectories from the same dataset with aligned origins (gray) and polar probability distribution of instantaneous swimming directions projected in the x-y plane (purple, solid line). A flat distribution (dashed) is shown for reference. For visual clarity, only those 7688 trajectories with a minimal duration of 1 s are shown, comprising 24,755 s of trajectory data. The polar distribution of orientations is based on the full dataset (9294 trajectories with duration 26,329 s).