Infection-mediated transport enables phage-bacteria coexistence in expanding populations

Bacteriophage infections of their bacterial hosts are central to the fate of the two most abundant organisms on the planet. Yet, the spatiotemporal balances leading to coexistence between lytic phages and bacteria are hard to predict. Intriguingly, chemotacting bacteria can pick up and co-propagate with virulent phage infections for extended periods, raising the question whether the emergence of this spatially complex persistence requires fine-tuning of phage-host interactions. Here we report that these travelling infections frequently settle into stable, linearly propagating waves for a variety of lytic phages and hosts, which coexist in 'Lytically Amplified Spatially Extended Regions' (LASERs). We propose a spatial model of phage-bacteria infection dynamics that recapitulates the empirical phage LASERs patterns, indicating that our observations are consistent with a simple combination of microbial growth, chemotaxis, and phage infection ingredients. We leverage this theoretical formulation to map features of phage-bacteria interactions into quantitative properties of the stable infection waves. We observe these propagating infections to be remarkably robust to environmental changes and bacteria front speed. The travelling infections stability offers avenues for long-distance transport of phage through unmixed environments much faster than viruses would otherwise diffuse.