Antagonism between toxin-secreting yeast strains as an experimental model for biological nucleation dynamics

Antagonistic interactions are widespread in the microbial world and affect microbial ecological and evolutionary dynamics. Microbial communities in the natural environment and within animal hosts often display spatial structure that affects biological interactions, but much of what we know about microbial antagonism comes from laboratory experiments performed with well-mixed communities. We manipulated two strains of the budding yeast Saccharomyces cerevisiae, expressing different "killer yeast" toxins, to independently control the rate at which they released their toxins. We developed mathematical models that predict the experimental population dynamics of antagonistic competition in both well-mixed and spatially structured populations. In both situations, we experimentally verified theory’s prediction that stronger antagonists can invade weaker ones only if the initial invading population exceeds a critical nucleation threshold. Finally, we found that toxin-resistant cells and weaker killers arose in spatially structured competitions between toxin-producing strains, suggesting that adaptive evolution can affect the outcome of microbial antagonism.