What does a lysis pattern tell about phage-bacteria co-propagation?

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Synopsis

Bacteriophages, or phages in short, are critical modulators of ecological and evolutionary processes in microbial communities, and for this to happen, phages and their host bacteria must maintain coexistence. In natural environments with limited nutrient supplies, motile bacteria can actively migrate towards locations of richer resources. Although phages are not motile themselves, they can infect motile bacterial hosts and spread in space via the hosts. Therefore, in a migrating microbial community coexistence of bacteria and phages implies their co-propagation in space. Here, we combine experimentation and mathematical modeling to explore how phages and their motile host bacteria coexist and co-propagate. When lytic phages encountered motile host bacteria in our experimental setup, a sector-shaped lysis zone formed. Our mathematical model indicates that local nutrient depletion and the resulting inhibition of proliferation and motility of bacteria and phages are key to formation of the observed lysis pattern. The model further reveals the straight radial boundaries in the lysis pattern as a tell-tale sign for coexistence and co-propagation of bacteria and phages. Emergence of such a pattern, albeit insensitive to extrinsic factors, requires a balance between intrinsic biological properties of phages and bacteria. This phenomenon likely both results from co-evolution of phages and bacteria, and ensures co-existence to allow co-evolution to happen.