Figure 3.

Steady-state distributions from experimental chemotaxis assays in single and opposing gradients of MeAsp and maltose along with the best fits obtained from parameter sweeps of SPECS with the transport-and-sensing model. Each experimental distribution of the relative concentration of bacteria was obtained using the bacterial positions measured over all channel locations and biological replicates (Materials and methods). In each plot, the maximal maltose concentration is at position x = 0 μm, and the maximal MeAsp concentration is at position x = 600 μm. Legends give the chemoattractant concentrations in the outer channels. To quantify each averaged response, we use the chemotaxis migration coefficient (CMC), which is defined as $\text{CMC}=\frac{\text{<x>}-300}{300}$, where ⟨x⟩ is the average position in microns of the cells across the 600-μm channel. (A) Single-gradient maltose experiments. CMC values = -0.022 (0.2 μM maltose), -0.22 (2 μM maltose), -0.33 (4 μM maltose), -0.25 (8 μM maltose), and -0.016 (20 μM maltose). (B) Single-gradient MeAsp experiments. CMC values = 0.030 (1.15 μM MeAsp), 0.16 (4.6 μM MeAsp), 0.55 (100 μM MeAsp), and 0.48 (460 μM MeAsp). (C) Opposing-gradient experiments with 2 μM of maltose in the left outer channel and various MeAsp concentrations in the right outer channel. CMC values = -0.26 (0.46 μM MeAsp), -0.14 (1.15 μM MeAsp), -0.061 (4.6 μM MeAsp), 0.45 (46 μM MeAsp), and 0.52 (460 μM MeAsp). (D) Additional opposing-gradient experiments with 2 μM maltose in the left outer channel and further intermediate MeAsp concentrations in the right outer channel. CMC values = -0.10 (2 μM MeAsp), -0.022 (4μM MeAsp), -0.021 (6 μM MeAsp), 0.032 (8 μM MeAsp), and 0.13 (10 μM MeAsp). To see this figure in color, go online.