Extended Data Fig. 7: Aspartate uptake and further analysis of expansion dynamics.
From: Chemotaxis as a navigation strategy to boost range expansion
a–c, Characterization of aspartate uptake for different growth conditions and strains. a, Aspartate uptake was determined using a colorimetric method to quantify remaining aspartate concentrations during growth (see Supplementary Text 1.2.3). In brief, change of aspartate concentration in the medium was measured during exponential growth at different cell densities (OD). Data are for growth with glycerol as the major carbon source (40 mM) and 0.8 mM initial aspartate concentration. Measurements for native aspartate uptake (wild type) and for different aspartate-uptake mutants (ΔgltJ and ΔgltP as well as the double mutant ΔgltJP). Lines show a linear fit. Uptake rate was determined by multiplying the obtained slope by the growth rate. b, The strains shown in a with different aspartate uptake rates exhibit similar growth rates. For each strain, the data points were collected from a single experiment. c, Dependence of aspartate uptake on growth rate for strains with native aspartate uptake. Growth is varied using wild-type cells (HE206, circles) grown on different sugar sources (acetate, mannose, glycerol or glucose), or by using the glpK* mutant (HE433, triangle) or the glycerol titration mutant (HE443, squares), in glycerol with different levels of the inducer 3MBA (25, 50 or 800 μM). Aspartate (0.8 mM) was provided in each case for the measurement of aspartate uptake (see a). Line shows linear fit with parameters specified in Supplementary Text 1.2.3. a, b, Data obtained for strains carrying fluorescence plasmids. c, Data obtained for non-fluorescent strains (two biological replicates, means shown). Data, strain information and medium conditions including concentrations of carbon sources are provided in Supplementary Table 7. d, Expansion dynamics with glucose as the primary carbon source. The dynamics of the front, shown to be described well by the GE model in Fig. 3b in glycerol with aspartate, is examined with the primary carbon source being glucose (20 mM). In the presence of 100 μM aspartate, the observed front propagation dynamics (purple squares) is correctly captured by the GE model again (purple line), by merely replacing the growth rate by that in glucose (λ = 1.0 h−1) with no additional adjustment of the chemotactic coefficient χ0. For reference, expansion is also shown for the condition in which no additional chemoattractant was provided (0 μM aspartate, open green squares), and the corresponding data for the condition in which glycerol was the primary carbon source (open black circles and corresponding lines from Fig. 3b). e, f, Dependence of expansion speed on attractant concentration with glycerol or glucose being the major nutrient. The increase in expansion speed at low attractant concentrations followed by decrease at higher concentrations, as previously observed43, is qualitatively captured by the GE model (dashed grey lines) in both cases. A better quantitative agreement between model and data is obtained when the linear growth term in the GE model (equation (1) in Fig. 3a) is changed to the logistic form λρ(1 − ρ/ρc). Here ρc is the carrying capacity, introduced to capture saturation of cell density in the front bulge (Supplementary Text 2.3). Predictions by the model are shown for different carrying capacities as coloured lines. In line with the strict requirement for oxygen when growing on glycerol and the observation that cells at high density accumulate at the agar surface when expanding with glycerol as major nutrient source (data not shown), the carrying capacity needed to resemble the observations is much lower for glycerol (e) than for glucose (f). Data points represent means of biological replicates (n = 2 or more) with error bars (s.d.) shown for n ≥ 3; see Supplementary Table 9 for data and sample sizes. g–i, Effect of varying aspartate uptake rate on expansion speed. The GE model predicts the expansion speed to be independent of the attractant uptake rate if all other parameters are kept fixed (g, dashed black line; Supplementary Text 2.2), with differences in attractant uptake compensated by changes in bacterial density at the front (h), such that the total rate of attractant depletion remains constant. This prediction was tested by characterizing the expansion dynamics of the aspartate-uptake mutants (strains HE506, HE552, HE555; Supplementary Text 1.1, Supplementary Tables 3, 7), which exhibited up to threefold difference in aspartate uptake (a), but only ~20% change in expansion speed (g). The small changes are readily accounted for by incorporating the small growth rate differences between these strains (b) into the GE model while keeping all other parameters fixed (g, green crosses). In addition, the aspartate-uptake mutants exhibited increasing peak densities at the front as predicted by compensation for reduced uptake (compare i, h). j–k, Predicted and observed changes in density profiles when varying the growth rate by titrating glycerol uptake in strain HE486 using 25 μM, 50 μM or 800 μM of the inducer 3MBA. For g, i, k, data were obtained from a single experiment for each strain and condition.
