Host range expansion of Shigella phage Sf6 evolves through the dual roles of its tailspike
The first critical step in a viruss infection cycle is attachment to its host. This interaction is precise enough to ensure the virus will be able to productively infect the cell, but some flexibility can be beneficial to enable co-evolution and host range switching or expansion. Like many bacterial viruses, bacteriophage Sf6 utilizes a two-step process to recognize and attach to its host, Shigella flexneri. Sf6 first recognizes the lipopolysaccharide (LPS) structure of S. flexneri, then binds to either outer membrane protein (Omp) A or OmpC. This phage typically infects serotype Y strains but can also form small, turbid plaques on serotype 2a2 with greatly reduced plating efficiency, suggesting inefficient infection. To examine the interactions between Sf6 and this sub-optimal host, phage were experimentally evolved using mixed populations of S. flexneri serotypes Y and 2a2. The recovered mutants could infect serotype 2a2 with greater efficiency than the ancestral Sf6, forming clear plaques on both serotypes. All mutations mapped to two distinct regions of the tailspike protein: 1) adjacent to, but not part of, the LPS binding site near the N-terminus; and 2) at the distal, C-terminal tip of the protein. Rather than weak interactions between the Sf6 tailspike and 2a2 O-antigen, LPS of this serotype appears to inhibit infection by binding the wild-type particles more strongly, effectively removing them from the environment. These mutations reduce the inhibitory effect by either reducing electrostatic interactions with the O-antigen or increasing reliance on the Omp secondary receptors.