Local chemotactic response of Escherichia coli in fluid and near surfaces

Bacteria can adjust their swimming behaviour in response to chemical variations, a phenomenon known as chemotaxis. This process is characterised by a drift velocity that depends non-linearly on the concentration of chemical species and its "local" gradient. To study this process more effectively, we optimised a 3-channel microfluidic device to generate a stable, linear concentration profile of chemoattractants. This setup allows us to monitor the response of Escherichia coli to casamino acids or α-methyl-DL-aspartic acid at the individual level. By analysing the movement of a population of individuals both in fluid and on surfaces, we achieve faster, more accurate quantification of the population's chemotactic response. In the fluid, the chemotactic response is described by the equation v_c = χ(c)∇c, with χ(c) = χ₀/[(1 + c/c_-)(1 + c/c₊)] the chemotactic susceptibility. For c_- ≪ c ≪ c₊, i.e. when bacteria perform chemotaxis, the bacterial chemotactic velocity is proportional to the concentration gradient divided by the concentration and v_c ∝ ∇c/c = ∇(log c). However, on surfaces, the chemotactic flux is inhibited.