Fig. 3.

Application of the framework to experimental systems (see Supplementary Movies 2–4). a The QS-based mutualism system. IPTG modulates stress and aTc induces QS-mediated mutualistic interaction. b Measurements of coexistence and collapse and corresponding δ values. Coexistence and collapse are measured by coculturing the two strains starting from the same initial densities. δ is measured by OD of M₂ monoculture after 32 h of culturing. c Empirical calibration of B(v). B(v) reveals how [IPTG] and [aTc] together modulate the effectiveness of the interaction. d B(v)/δ is predictive of coexistence versus collapse and total final density. The x-axis range of [0.5–1.5] is used to highlight the transition (this also applies to other prediction plots). The trend continues to hold beyond this range. The y-axis represents normalized final cell density. e The pairwise yeast auxotroph system. The growth of both auxotrophs are suppressed in monocultures. With increasing Trp and Leu supplemented to the co-culture, the growth suppression can be alleviated. f The amount of supplemented amino acid and ratio of initial densities modulate system behavior. Only [Trp] is shown and [Leu] is eight times of [Trp]. The total initial density of the two strains are kept constant. Corresponding δ values are measured based on growth yield of $\Delta Leu$ monocultures, assuming δ is independent of initial density. g Optimal effective benefit occurs at an intermediate ratio of initial density. h B(v)/δ is predictive of normalized total cell number per culture well. i The 91 mutualism systems constructed by 14 engineered E. coli auxotrophs. Growth suppression is evident in their inability to survive individually in minimal medium. However, two auxotrophs can potentially survive through mutualistic interaction in a co-culture by exchanging amino acids. j System outcomes for all 91 pairs and δ for each of the 14 auxotrophs. Note that for one pair, the calibration is done twice with δ of either strain. k The calibrated B(v) for E. coli auxotroph systems. l B(v)/δ is predictive of the normalized fold change of final total density relative to initial density