Phase variation in Bacteroides fragilis governs susceptibility to a microvirus and drives its evolution

• We isolated the first Bacteroidota-infecting Microviridae phage, φHBP1. • B. fragilis likely exploits phase variation to be resistant against φHBP1. • φHBP1 evolves to reinfect the resistant population by mutations in the capsid and pilot proteins The interaction and co-evolution between human gut bacteria and their phages shape the dynamic gut microbiome, exerting a significant impact on human health. However, the underlying mechanisms are largely unexplored. In particular, a bacteria-phage interaction model of the Bacteroidota phylum and the Microviridae phages is lacking, limiting our understanding of their ecological roles in human gut. In this study, we isolated a Bacteroidota-infecting Microviridae phage φHBP1 from human feces. Infection of its host Bacteroides fragilis with φHBP1 drives multiple genomic structural variations, which are correlated with host resistance to φHBP1. In turn, our phage evolution assay in B. fragilis H1 obtained φHBP1 mutants that carry mutations within the capsid and pilot proteins and can reinfect the resistant bacterial population. Together, our findings provide novel insights into an antagonistic co-evolution mechanism between gut phage and bacteria, and hold important implications for diversifying phages through evolution to target resistant bacteria in phage therapy.