Fig. 4.

Metabolic profile of the culture medium and emergence of gradients. (A) Extracellular metabolites secreted in the culture medium by WT cells grown without shaking with ribose as the sole carbon source for 60 h. Metabolites were analyzed by liquid chromatography–mass spectrometry (SI Appendix, Supplementary Text). (B) Time dependence of densities (OD₆₀₀) of WT (solid symbols) or ΔcheY (open symbols) cells grown individually without shaking, with ribose as the sole carbon source, in the top (circles), middle (squares), and bottom (triangles) layers of the liquid culture. **P < 0.05, ***P < 0.01 in 2-tailed t test. (C) Proportion of WT cells in different layers of the cocultures with ΔcheY cells grown for 72 h without shaking with indicated carbon sources. (D) Full 2D simulations of competition between the WT and ΔcheY cells cocultured on different primary nutrients and excreting secondary nutrients as detailed in SI Appendix, Supplementary Text. Shown are simulations for chemotactic response to only primary nutrient (black open diamonds), to both primary and secondary nutrients (blue open triangles), and to only secondary nutrient (green open circles), with chemotactic sensitivity varying dependent on the carbon source as in SI Appendix, Fig. S1C. Also shown is the simulation with chemotactic response to both primary and secondary nutrients but with constant high sensitivity of chemotaxis, simulating motility induction under P_tac promoter (red open squares). (E) Spatial maps of WT cell density (Top, in units of OD) and of concentration of secondary nutrient s (Bottom, in millimolar), for indicated time points (t in hours) and for N-value of the primary nutrient 0.3 h⁻¹.