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. 2024 Sep 2;9(9):2308–2322. doi: 10.1038/s41564-024-01729-3

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© The Author(s) 2024

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — a, We used Taylor–Aris dispersion to generate concentration gradients along a 2 m long tube, which then flowed past surface-attached cells in microfluidic devices. We filled the system with media containing succinate (C_MAX = 1.16 mM). At t = 0, media containing a lower succinate concentration (C_MIN = 0.84 mM) was pulled through the system. As fluid moves fastest along a tube’s centreline, a plug of lower concentration fluid forms (panel i) but is rapidly mixed across the tube width via molecular diffusion (panel ii). The fluid interface forms a longitudinal gradient with an ~1.6 m length scale such that surface-attached cells within the device experience smooth temporal decreases in concentration (panel iii to panel iv). b,c, Using dye, we quantify succinate concentration (b) and temporal concentration gradient over time (c) (blue lines; dashed green lines show a control with 1 mM succinate throughout). Cells experience approximately the same mean temporal concentration gradient that cells experience in dual-inlet chemotaxis experiments (Extended Data Fig. 1 and ref. ³³), but with ~16,000-fold smaller spatial gradients. d, In the 1 h period before the succinate gradient entered the device (interval t₁), cell reversal rates were statistically indistinguishable between experiment (white bar, blue outline) and control (white bar, green outline; one-sided exact Poisson test (Methods) yielded P = 0.316). Similarly, reversal rates in the presence of a temporal succinate gradient (interval t₂; light grey bar, blue outline) and in the 1 h period after the gradient had cleared the microfluidic device (interval t₃; dark grey bar, blue outline) were statistically indistinguishable from the reversal rates in the control (P = 0.842 and P = 0.368). The number of reversals observed was n_r = 1,496 and 1,391 across n_t = 468,596 and 439,632 trajectory points in the control and experimental conditions, respectively. Error bars show 95% confidence intervals about the mean reversal rates assuming that reversals follow a Poisson distribution (Methods). Data shown here are representative of two bio-replicates (Extended Data Fig. 5). Source data provided as a Source data file.

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