The spatial and metabolic basis of colony size variation

Spatial structure impacts microbial growth and interactions, with ecological and evolutionary consequences. It is therefore important to quantitatively understand how spatial proximity affects interactions in different environments. We test how proximity influences colony size when either Escherichia coli or Salmonella enterica are grown on different carbon sources. The importance of colony location changes with species and carbon source. Spatially-explicit, genome-scale metabolic modeling predicts colony size variation, supporting the hypothesis that metabolic mechanisms and diffusion are sufficient to explain the majority of observed variation. Geometrically, individual colony sizes are best predicted by Voronoi diagrams, which identify the territory that is closest to each colony. This means that relative colony growth is largely independent of the distance to colonies beyond those that set territory boundaries. Further, the effect of location increases when colonies take-up resource quickly relative to the diffusion of limiting resources. These analyses made it apparent that the importance of location was smaller than expected for experiments with colonies growing on sugars. The accumulation of toxic byproducts appears to limit the growth of large colonies and reduce variation in colony size. Our work provides an experimentally and theoretically grounded understanding of how location interacts with metabolism and diffusion to influence microbial interactions.