Phage infection fronts trigger early sporulation and collective defense in bacterial populations

I. ABSTRACT Bacteriophage (phage) infect, lyse, and propagate within bacterial populations. However, physiological changes in bacterial cell state can protect against infection even within genetically susceptible populations. One such example is the generation of endospores by Bacillus and its relatives, characterized by a reversible state of reduced metabolic activity that protects cells against stressors including desiccation, energy limitation, antibiotics, and infection by phage. Here we tested how sporulation at the cellular scale impacts phage dynamics at population scales when propagating amongst B. subtilis in spatially structured environments. Initially, we found that plaques resulting from infection and lysis were approximately 3-fold smaller on lawns of sporulating wild-type bacteria vs. non-sporulating bacteria. Notably, plaque size was reduced due to an early termination of expanding phage plaques rather than the reduction of plaque growth speed. Microscopic imaging of the plaques revealed ‘sporulation rings’, i.e., spores enriched around plaque edges relative to phage-free regions. We developed a series of mathematical models of phage, bacteria, spore, and small molecules that recapitulate plaque dynamics and identify a putative mechanism: sporulation rings arise in response to lytic activity. In aggregate, sporulation rings inhibit phage from accessing susceptible cells even when sufficient resources are available for further infection and lysis. Together, our findings identify how dormancy can self-limit phage infections at population scales, opening new avenues to explore the entangled fates of phages and their bacterial hosts in environmental and therapeutic contexts.