Stable coexistence and transport of lytic phage infections with migrating bacterial hosts

Bacteriophages (phages), viruses that exclusively infect bacteria, coexist with their bacterial hosts across diverse environments at densities exceeding 10⁷ per milliliter in marine surface waters, 10⁸ per milliliter in soils, and 10⁹ per milliliter in the human gut. In contrast, phage lysis of bacterial populations within well-mixed in vitro environments select for the emergence of phage-resistant bacterial mutants, which, in turn, select for host-range expansion phage mutants and, typically, the collapse of phage populations altogether. This gap in outcome raises a question: What enables long-term phage-bacteria coexistence? Here, we show how interactions in space can facilitate stable coexistence and long-range transport of virulent phage along with migrating bacteria. Combining theory, simulation, and experiments across multiple phage-bacteria systems, we reveal a chemotaxis-driven mechanism that robustly stabilizes coexistence and dispersal of virulent phages with migrating hosts. These findings suggest the ecological relevance of spatial interaction mechanisms that reinforce stability between antagonistic partners in the absence of perpetual cycles of defense and counterdefense, which may be broadly applicable across phage-bacteria systems.