Webervirus phages exploit capsule- and O-antigen-targeting receptor-binding proteins to access diverse Klebsiella pneumoniae hosts

Klebsiella pneumoniae frequently downregulates or completely loses its capsule. This abolishes infection by capsule-targeting phages but concomitantly may favor infection by phages that exploit non-capsular receptors. Although capsule-targeting mechanisms have been studied in detail, the infection strategies of non-capsular phages remain poorly understood. Here, we analyzed receptor usage across a collection of Webervirus phages and found two contrasting phage-host interaction modes within Webervirus: one group relies on the capsular polysaccharide (CPS) to mediate productive infection, whereas infection by the other group is inhibited by the capsule, compellingly dependence on non-capsular receptors. Genome comparisons revealed a conserved morphogenesis backbone but highly variable receptor-binding protein (RBP) genes among Webervirus phages. These RBPs clustered into distinct clades that correlated with CPS-dependent and non-CPS receptor usage. Within the non-capsular clade, host genetics and complementation established the lipopolysaccharide O-antigen as the required receptor for these phages. Subsequent functional characterization of the RBPs confirmed that this clade specifically targets the O-antigen. Finally, exchanging RBPs by homologous recombination was sufficient to switch receptor usage, and combining phages with distinct receptor usage improved growth suppression in a representative clinical isolate. Together, these results define receptor-linked infection modes within Webervirus and show that RBP variation provides a modular route to diversify host range and increase robustness against capsule-state variability.

Klebsiella pneumoniae is a major multidrug-resistant pathogen, and its frequent capsule loss in clinical settings strongly affects phage therapy outcomes. Our work reveals two contrasting infection modes within Webervirus phages-capsule-dependent versus capsule-inhibited-and links each mode to distinct receptor-binding protein (RBP) clades. By identifying the O-antigen as a key non-capsular receptor, functionally validating O-antigen-targeting RBPs, and showing that RBP exchange can switch receptor usage and that combining complementary phages improves suppression of a clinical isolate, we provide a modular framework for rationally designing phage cocktails that remain effective despite capsule-state variability.