Figure 4

Interactions between expansion fronts. (A) Kymograph of fluorescence intensity for a habitat where a stable boundary is observed. (B) Enlarged view of panel A, for the 6 patches centered at the interface between the green and red populations at t = 19 h. (C) Enlarged view of the 6 patches at the left end of the habitat shown in A at t = 19 h. A few red cells are indicated by the white arrows in the inset. (D) Enlarged view of the 6 patches at the right end of the habitat shown in A at t = 19 h. (E) Kymograph of fluorescence intensity where the green population is expelled from the habitat by the red population, before the two fronts come into physical contact. (F) Kymograph of fluorescence intensity where the green population is expelled from the habitat by the red population, the inset shows that there has not been any physical contact between red cells and the green front before the latter changes direction. Note how the leading edge of the green front and the high density region, roughly 8 patches behind the leading edge, change direction almost simultaneously (G) Kymograph of fluorescence intensity for a habitat inoculated at both sides with a single mixed culture of strains JEK1036 (green) and JEK1037 (red), note that the change in color from red to green with increasing time is mostly due to changes in the fluorescence intensity per cell of RFP compared to GFP and not due to de-mixing of the strains as can be seen by comparing their occupancy patterns (Additional file 7A).