Figure 1. E. coli evolves faster migration through a porous environment in rich and minimal media.

(a) A schematic of the selection procedure. E. coli are inoculated into the center of a low viscosity (0.3 % w/v) agar plate where they form an expanding colony driven by metabolism and motility. After a fixed period of incubation, samples are taken from eight locations around the outer edge of the expanded colony, mixed, and used to inoculate a fresh plate. (b) Shows expanded colonies in rich medium (LB) plates after 12 hr of incubation over five successive rounds of selection. The color bar to the right applies to all panels in (b), with darker gray indicating higher cell density. Image intensity is assumed to be monotonic but not linear with cell density in the plate. Scale bar in the left panel is 1 cm and applies to all panels in (b). (c) Shows the rate of migration as a function of round of selection over 15 rounds for five replicate selection experiments in rich medium. No rate is reported for replicate 1 round 8 due to failure of the imaging device. Errors in measured rates of migration are smaller than the size of the markers. (d) Shows colonies (gray regions) in minimal medium (M63, 0.18 mM galactose) after 48 hr of incubation. The color bar to the right applies to all panels in (d). The scale bar in the left panel is 1 cm. (e) Shows the rate of migration as a function of round of selection over 10 rounds for five replicate selection experiments in minimal medium. Errors in migration rates were smaller than the size of markers. See Materials and methods for details of image processing in both experiments.

DOI: http://dx.doi.org/10.7554/eLife.24669.003

Figure 1—figure supplement 1. Selection with non-chemotactic ($\mathrm{\Delta }$cheA-Z) mutant.

Front migration rates of non-chemotactic mutants in 0.3 % w/v agar at 30°C with LB (left panel) and M63 0.18 mM galactose (right panel). Errors are smaller than the size of the markers, except for the red replicate in rich medium at round 2. Red and black correspond to two independent selection experiments. Note the vertical scales. In minimal medium, zero migration rate denotes plates where density increased in the vicinity of the site of inoculation but no migration was observed. In these cases no measurable migration rate was obtained.

DOI: http://dx.doi.org/10.7554/eLife.24669.004

Figure 1—figure supplement 2. Change in migration rate during long-term liquid culture.

(left) The founder strain (Figure 1c, main text, $s=$ 0.3 $\pm$ 0.01 cm h⁻¹) was inoculated into a turbidostat and continuously cultured in LB at 30°C for approximately 200 generations. Samples were periodically drawn from the turbidostat and used to inoculate 0.3 % w/v agar LB plates in duplicate. Migration was recorded via webcam as described in the main text. Error bars are standard errors from regression of radius with time. Note the scale on the y-axis. (right) Identical experiment in minimal medium conditions. Founding strain was grown in a single chemostat (doubling time 5.7 hr) in minimal medium for 120 generations. Plates were inoculated from samples drawn from the chemostat, two plates at each time point for the first four time points and then one plate at each time point. The last four time points (where the rate appears to saturate) exhibit a slower migration rate than the round 10 migration rates in Figure 1e ($p=0.02$).

DOI: http://dx.doi.org/10.7554/eLife.24669.005

Figure 1—figure supplement 3. Adaptation in rich medium depends on sampling location.

Migration rate as a function of the round of selection. Colored traces are reproduced from Figure 1 in the main text. Black circles and squares are two replicate selection experiments where populations are sampled halfway between the center of the colony and the outer edge after each round of selection.

DOI: http://dx.doi.org/10.7554/eLife.24669.006

Figure 1—figure supplement 4. Comparison of founding and evolved strains to RP437.

Single-cell swimming in rich medium: (left) Run duration distributions identical to those shown in Figure 3a–b of the main text. 77 RP437 individuals were tracked from a culture at the same optical density as founder and round 15 (replicate 1). A total of 9218 run events were recorded. The average $\pm$ standard deviation in run duration for RP437 is 0.76 $\pm$ 0.82 s. (right) Comparison of run speeds for the same three strains. RP437 has an average $\pm$ standard deviation in run speed of 18.6 $\pm$ 6.4 μm s⁻¹. The average run duration for RP437 exceeds that of round 15 (${\displaystyle p<{10}^{-4}}$), and the average run speed is smaller than that of round 15 (${\displaystyle p<{10}^{-4}}$). For the RP437 strain in rich medium, we measure a migration rate of 0.15 $\pm$ 0.01 cm h⁻¹ and a liquid culture growth rate of 1.1 $\pm$ 0.02 h⁻¹.

DOI: http://dx.doi.org/10.7554/eLife.24669.007

Figure 1—figure supplement 5. Persistence of rich medium fast migrating phenotype in liquid culture.

A strain isolated after 15 rounds of selection in rich medium (Figure 1c, replicate 1, main text, $s=$ 0.6 cm h⁻¹) was inoculated into a turbidostat and continuously cultured in LB at 30°C for approximately 140 generations. The number of generations was estimated assuming a constant generation time of 36 min. Samples were periodically drawn from the turbidostat and used to inoculate 0.3 % w/v agar LB plates. Migration was recorded via webcam as as described in the main text. Error bars are standard errors from regression of radius with time.

DOI: http://dx.doi.org/10.7554/eLife.24669.008