Extended Data Fig. 2: Swimming characteristics in liquid media (well-mixed conditions, no gradients).
From: Chemotaxis as a navigation strategy to boost range expansion
a–d, Average swimming speed (purple circles) and the fraction of motile cells (grey squares) were characterized for cells taken from batch cultures along a growth curve at different optical densities (OD600; green triangles). For each condition, data points were collected from a single experiment. a, Culture was grown in LB medium, starting with an overnight LB culture that was sitting in saturation for 18 h before dilution into fresh LB medium at time zero. This experiment was essentially a repeat of previous work14,15; similar results were obtained, with motility increasing as growth progressed. Our data in the following panels suggest that most of the increase in swimming speed resulted from the increased fraction of motile cells in the first 2 h. b, Culture was grown in minimal medium with 10 mM glycerol and 1.7 mM aspartate, starting with an overnight pre-culture (same medium) that was in saturation for about 18 h before inoculation into fresh medium (time zero). As observed for LB (a), it took several hours for both the motile fraction and swimming speed to recover. c, Culture was grown in LB medium continuously for 10 generations, with bacterial density always kept below OD600 = 0.5 before dilution to fresh LB at time zero. Both the motile fraction and the swimming speeds are high in the exponential growth phase (0–2 h) except for a dip at an OD600 of approximately 0.5. Swimming speed and motile fraction decreased after the stationary phase was reached. d, Culture was grown in the same minimal medium (glycerol + aspartate) for about 20 generations, with bacterial density maintained below OD600 = 0.6 before measurement. As with LB (c), swimming speed and motile fraction remained high in the exponential growth phase (0–4 h) before sharply decreasing after entering the stationary phase. The strong variation of the fraction of motile cells observed here is in line with previous observations on cell-to-cell variation in swimming behaviour68,69 and can strongly affect the dynamics of migrating populations24,70. e, f, Swimming behaviour observed in steady-state growth (as in d for the first ~3 h) for different (relatively fast) growth conditions and different E. coli strains (Supplementary Table 5). Swimming speeds (v) and durations between tumbling events (τ) obtained from trajectory analysis are shown in e and f, respectively. Black symbols show results for the NCM3722B-derived strains (HE206, HE433, HE443; growth at 37 °C) mainly used in this study. A similar weak dependence of quantities on growth was observed for MG1655B (red symbols, growth at 37 °C) and RP437 (blue symbols, growth at 30 °C). g, Estimated effective diffusion coefficient, D = v2τ, for the different growth conditions in NCM3722B-derived strains. e–g, Data points show the means of two biological replicates for HE433 and HE443 and the results of single experiments for HE206, MG1655B and RP437. See Supplementary Text 1.2, 1.3 for methods, Supplementary Table 5 for data values and conditions, and Supplementary Text 1.1 for strain details.
