Spatial heterogeneity influences phage-host co-existence dynamics and phage resistance development in Vibrio anguillarum

Exposure of bacterial populations to phage infection pressure is a significant driver of phage-host co-evolution and diversification. However, the impact of the spatial organization of bacterial population co-existence and co-evolution dynamics remains poorly understood. Here, we investigated how the spatial structure of the host population affects phage-host interactions by co-cultivating a Vibrio anguillarum strain with the lytic T4-like vibriophage KVP40 under conditions that resulted in either a homogenous, well-mixed population or a heterogeneous population with bacterial aggregates. We observed markedly different temporal dynamics associated with the two population structures over a 30-day adaptive laboratory evolution experiment. Phage and host dynamics suggested that phage-sensitive subpopulations retained in the aggregates substantially prolonged the coexistence of phages and bacteria, relative to the homogenous environment. However, the limited phage propagation on the sensitive subpopulations was insufficient for host range expansion to occur. In contrast, daily supplementation of sensitive host bacteria in parallel experiments readily led to the emergence of phage mutants with enhanced infective capability and an expanded host range. These results underscore the importance of bacterial spatial organization in determining the outcome of phage attacks and highlight the complex interplay between genomic processes and ecological conditions in driving evolutionary innovations.