Differences in bacteria killing rates are mitigated by finite size effects

Bacteria often live in densely packed, surface attached communities called biofilms in which different species and strains compete for space and resources. Many bacteria have evolved methods of killing competitors, and a wide range of killing abilities have been empirically observed both in type and efficiency. Typically, biofilm competition is studied on agar plates; in these experiments, there are effectively no spatial limitations on biofilm growth, and biofilms may contain millions of cells or more. However, in nature biofilms frequently grow in confinement; geometric constrains in these environments limit the size of biofilms. To determine the impact of finite size effects on deadly competitions within biofilms, we perform individual based simulations of bacteria confined to different sized environments. We find that, if both strains can kill each other, the fitness advantage of the superior killing strain decreases as the biofilm becomes more spatially limited. These results suggest that bacteria that primarily live in small groups may have little incentive to evolve to be better killers, which may partially explain the wide range of observed killing efficiencies.