The worst epidemic of cholera in recent history between 2010 and 2013 in Haiti caused the death of more than 8,000 Haitians. While Vibrio cholera is mainly considered as the responsible pathogenic agent, the actual pathogen is a temperate phage that upon successful infection of its Vibrio host turns the bacterium into a fatal disease. By integrating their DNA into the bacterial genome bacteriophages can transform an apparently stable commensalism between Vibrio spp. and animal hosts into a highly virulent infection making phages one of the most important drivers of virulence evolution across the animal kingdom. However, such three-way interactions are often very complex and still poorly investigated.

Here I present first results from a three-way species interaction using a well-established model system consisting of pipefish, Vibrio spp. and their temperate phages. In a cross-infection matrix of 75 Vibrio and 75 temperate phages three distinct groups of Vibrio bacteria were found, i.e. (1) Vibrio that are susceptible to generalist and specialist phages, (2) Vibrio that are only susceptible to specialist phages and (3) Vibrio that are resistant against any phage infection. To further gain an integrative view across all three levels within this three-way interaction, we infected juvenile pipefish with bacteria stemming from one of the three groups. By measuring gene expression, mortality and infection intensity first results show that Vibrio, which are resistant to phages have a reduced infection intensity on pipefish. This indicates that bacteria virulence towards pipefish is significantly dependent on Vibrio susceptibility to phages, which emphasize the great importance of bacteriophages in determining Vibrio virulence.

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