Figure 2.

Experimental set-up and measurement of chemotactic fluxes. (a) Top view of the experimental sample chamber made of two reservoirs, containing attractant concentrations c = c₀ and c = 0 (or here, fluorescein, appearing green). The reservoirs are in contact through a narrow channel in the centre (box region). (b) The channel (length L, width w) at higher magnification, observed under fluorescence microscopy (×10), 1 h after making the chamber. The fluorescence intensity is proportional to chemical concentration. (c) The chemical (fluorescein) concentration profile in the middle of the channel is linear and stabilized in t_w = 2–3 h after filling the chamber. The concentration gradient ∇c and background concentration were calibrated as a function of c₀, L and w (see the electronic supplementary material, figure S2 and text). (d) R(t) of E. coli bacteria at t_w = 3 h, under different conditions but where the relative gradient is held approximately constant. The measured trajectories are linear functions of time. The inset shows part of a typical image from a movie sequence, with sides of length 180 µm. (e) Measured chemotactic velocity as a function of t_w for the same experiments as (d). The chemotactic velocity is nearly constant as a function of t_w and increases as a function of . The chemotactic velocities are normalized by the average swimming speed of the bacteria v₀ and the fraction of swimmers α, as described in the main text. The key is common to (d,e).