FIGURE 1.

Experiments to determine the chemotactic sensitivity χ₀ of E. coli. (a) Schematic of the microfluidic channel. Chemoattractant and fluorescein were injected in the microcapillary via inlet C by means of a passive valve. (b) Flow in the main channel (from A to B) was used to transport E. coli past the mouth (M) of the microcapillary, where a fraction of the population had swum into the microcapillary. Each white path is an E. coli trajectory. The image is a superposition of 200 frames captured over 6.2 s. (c and d) Epifluorescence images (using a 2× objective) of the microcapillary, initially filled uniformly with α-methylaspartate (t = 0; c), and later exhibiting a nonuniform concentration profile (t = 45 min; d). The latter was used to probe the chemotactic response of the E. coli cells that had swum into the microcapillary. 100 μM fluorescein was added to variable concentrations of α-methylaspartate (0.1, 0.5, or 1.0 mM) for visualization. (e) Trajectories of E. coli from 300 frames recorded over 9.4 s using a 20× objective. (f) Concentration profile C(x) obtained from d and normalized by the initial concentration C₀ in the microcapillary. The field of view is the same as in e.