A Novel Bacteriophage Capable of Jumping Between Marine and Freshwater Hosts

Understanding how viruses evolve when moving from host to host is an important area of study in ecology and epidemiology. Previously, a novel bacteriophage, Xuper, was isolated and characterized using the host Rhodobacter capsulatus, a bacterium that lives in freshwater environments. This bacteriophage was particularly interesting due to many distinguishing features: a unique prolate capsid, a DNA genome that is the largest of any known R. capsulatus phage, and the fact that it has no known relatives in our collection of viral isolates. However, we recently discovered that Xuper does have a remarkable similarity to two bacteriophages that infect Ruegeria pomeroyi, a bacterium that lives in marine environments. In examining the ability of Xuper to also infect this alternative host, we found that although it was capable of infecting this host, it was significantly less efficient in doing so. Through sequential plating experiments, we generated Xuper mutants that were better adapted for infecting the marine host than their freshwater isolation host, and the mutations responsible for these changes were identified using DNA sequencing. These findings will help to elaborate on mechanisms of host switching, phage evolution, and may begin to help us define functions for newly discovered phage genes