Fig. 2: Laboratory results show the coexistence of competing partner strains in a multiple mutualist community.

a Schematic showing the interactions between the E. coli partner mutualists that comprise the partner guild and the S. enterica shared mutualist. The arginine auxotroph, E arg− (“Er”), and the methionine auxotroph, E met− (“Em”), consume lactose and produce acetate. S met+ arg+ (“Smr”) consumes acetate and produces both arginine (arg) and methionine (met). b The growth rates in monoculture of the three strains differ (one-way ANOVA: F(2, 9) =  9897, P = 9.1e-16). Er grows faster than Em (Tukey HSD: P = 0.037), and Smr grows much more slowly than either E. coli strain (Tukey HSD: P < 1e-7). Em (n = 3) and Er (n = 3) were grown in lactose media with excess methionine and arginine, and Smr (n = 6) was grown in acetate media. All data points are shown, and the horizontal lines show means. Replicates were independent biological replicates performed over one experiment. c Growth rates of Em+Smr (n = 3) and Er+Smr (n = 3) cocultures. Em+Smr cultures grow faster (Welch’s two-sample t test: t(2) = 15, P = 0.004). All data points are shown, and the horizontal line shows the mean. Replicates were independent biological replicates performed over one experiment. d A mutual invasibility experiment with cocultures of Em and Er in a lactose medium with excess amino acids. The frequency of Em decreases from all starting frequencies, including when started at 0.6% of the population (inset plot; Welch’s two-sample t test: t(2) = 48, P = 0.0004), indicating that Em is the weaker competitor for lactose. The change in Em frequency was calculated as [Em/(Em + Er)]_final − [Em/(Em + Er)]_initial. Supplementary Fig. 4 shows that Em frequency also decreases when Smr is present in this environment. e A mutual invasibility experiment in the three-strain multiple mutualist community in a lactose medium. Em increases in frequency when started rare (Welch’s two-sample t test: t(2) = −58, P = 0.0003), but decreases in frequency when started common (Welch’s two-sample t test: t(2) = 6, P = 0.02), indicating that the two E. coli strains coexist. The change in Em frequency was calculated as [Em/(Em + Er)]_final − [Em/(Em + Er)]_initial. Smr yields are similar across all Em frequencies (Supplementary Fig. 7). In panels d and e, shapes indicate independent experimental batches (A = circles, B = triangles, C = squares). Sample sizes, means, and standard deviations are given in “Results”. Source data are provided as a Source Data file.