Fig. 5: Resource competition can lead to non-additive effects during simultaneous application of two antibiotics, with antagonism more likely than synergism.

a A community for which antibiotic targeting leads to antagonism. Before antibiotic treatment, all three species coexisted, as shown by the simplex of the resource consumption rates of three species (colored circles) along with the supply rates of three resources (star) with distances \({D}_{N}\), \({D}_{T1}\), \({D}_{T2}\) as in Fig. 4a, as well as the distance between the two targeted species \({D}_{T}\). b (Top) When the death rate of the orange species in (a) was increased, remapping of the convex hull resulted in extinction of the orange species. (Bottom) When the death rate of the blue species was increased, the blue species similarly went extinct. c When both species in (a) were targeted simultaneously, competition between the targeted species was relieved. (Top) Remapping due to both antibiotics led to only small changes in the consumption rate vectors compared with each antibiotic alone, as compared to unperturbed community in (a). (Bottom) Coexistence of all three species was restored, representing antagonism. d Like (a), but showing a community for which antibiotic targeting led to synergism. e Targeting of the orange (top) or blue (bottom) species in (d) alone preserved coexistence of all three species. f When both species in (d) were targeted simultaneously, the non-targeted species (green) outcompeted the targeted species, representing synergism. g The fraction of synergism (red) and antagonism (blue) for communities of the form in (a) and (d) (i.e., \({D}_{T1}={D}_{T2}\) and \({d}_{1}={d}_{2}\in \left(0,1\right)\)), averaged across all combinations of \({D}_{T2}\), \({D}_{N}\), and \({d}_{2}\). Synergism was more likely for high \({D}_{T1}\) and low \({D}_{N}\), and vice versa for antagonism. h Same as (g) but averaged across all values of \({D}_{T1}\), \({D}_{T2}\), and \({d}_{2}\). i Schematic of communities with consumption rate vectors drawn uniformly from the simplex, both death rates drawn independently from a uniform distribution between 0 and 1, and resources supplied at equal rates. \({D}_{N}\) and \({D}_{T}\) are labeled for a random community. j Of the communities in (i) without symmetry in the consumption rates of the targeted species, a smaller fraction exhibited synergism than antagonism.