Death & Chemotaxis: Uncovering the Dynamics of Bacterial Collective Migration in the Presence of Bacteriophages

Phages are viruses that infect and kill bacteria, shaping microbiomes and influencing their functions with critical implications for agriculture, food, and medicine. In nature, bacteria and phages coexist in crowded 3D environments, such as soil and biological tissues, which can impact their interactions, function, and collective behaviors. However, laboratory studies typically use well-mixed cultures or colonies on agar, which do not fully capture nature’s complexities. Here, we address this knowledge gap by directly visualizing phage-bacteria interactions in transparent, crowded matrices composed of packed hydrogel microparticles. Using this setup, we ask: how do interactions with phages influence the spatial organization and collective behaviors of bacterial populations in such complex environments? Through microscopy, we reveal the dynamics of motile Escherichia coli populations encountering lytic T4 phages, as a function of the initial spatial distributions and concentrations of both bacteria and phages. Our findings show that bacteria can take advantage of collective chemotaxis to traverse phage-rich environments without entirely relying on phage resistance. We develop a theoretical model that predicts our experimental observations, thereby establishing quantitative biophysical principles to understand phage-bacteria interactions in complex environments. Altogether, our work provides new insights that bridge the gap between simplified laboratory studies and natural settings.