Phage and bacteria: Competition and coexistence*

Bacteria and the viruses that infect them (phage) have been engaged in an epic struggle for billions of years. While the competition between phage and bacteria is intense---driving the evolution of systems such as CRISPR and restriction enzymes---the persistent diversity of these organisms is equally striking. Despite the presumption that a single resource should only support a single competitor, phage of different infection strategies preying on the same bacteria frequently coexist in the same environment; similarly, bacteria carry a wide and variable array of defense systems all targeting the same phage species. What leads to this continued diversity on both the single-organism and ecological scales? I will describe a mathematical model demonstrating that phage coexistence is a natural outcome of chaotic dynamics arising from competition among multiple phage and their bacterial hosts. I will then further develop this model to probe the costs and benefits of individual defense systems in bacteria, exploring how ecological and evolutionary dynamics affect bacterial heterogeneity. These models suggest that the multi-scale dynamics inherent to phage/bacteria competition paradoxically contribute to the remarkable diversity observed in nature.