Collapse and contingency in phage infections of migrating bacterial populations

Natural bacterial populations are subject to constant predation pressure by phages. Since phage are non-motile perhaps the simplest defense against phage is for bacteria to outrun their predators. In particular, chemotaxis may help the bacteria escape slowly diffusing phages. Here we study phage infection dynamics in migrating bacterial populations driven by chemotaxis. We find that expanding phage-bacteria populations support two migrating fronts, an outermost "bacterial" front driven by nutrient uptake and chemotaxis and an inner "phage" front at which bacterial population collapses due to phage infection. We show that with increasing adsorption rate and initial phage population, the rate of migration of the phage front increases, eventually overtaking the bacterial front and driving the system from a regime where bacteria outrun a phage infection to one where they must evolve phage resistance to survive. We suggest that this process requires phages to hitchhike with the migrating bacterial front by repeatedly re-infecting the fastest moving bacteria. A deterministic model recapitulates the transition. Our work opens a new, spatiotemporal, line of investigation into the eco-evolutionary struggle between bacteria and their phage predators.