The goal of this project is to understand the factors that promote the spread, invasion and establishment of the Vibrio parahemolyticus pathogenic ST36 clonal complex of strains.The two northernmost populations of ST36 in the Atlantic were each founded by unique but related individuals, that acquired different filamentous phage (Vipa26 and Vipa36), and each founder subsequently expanded their populations in these important shellfish aquaculture areas. These phage are highly similar to the f237 phage that is exclusively found in the pandemic clonal complex of strains that successfully spread across the globe in the mid-1990's, increasing the prevalence of Vp infections in all regions where it spread. This suggests to us that the phages may impart attributes that enhance invasiveness and successful translocation. The fitness of ST36 chronically infected with Vipa26 and Vipa36 is dependent on the yet undefined cost of harboring the phage weighed against the as yet undetermined benefits of harboring phage. Benefits could include improved competitive growth with bacteria that are outright killed or sustain a greater cost upon infection, anti-predation by protists, or enhanced accumulation in oysters. We expect to experimentally examine the cost-to-benefit balance of phage for the first time in Vp. We will quantify potential differential costs in terms of growth and viability in objective 1 and evaluate the contribution of phage to ST36 competitive fitness that underlies invasiveness in the Northeast in objective 2. The proposed experiments aim to test the specific hypothesis that chronic infection by Vipa26 and Vipa36 enhances the competitive dominance and/or virulence of ST36. We anticipate that if our hypothesis is true, that ST36 could withstand infection better and sustain a lower fitness tradeoff than some competing Vp strains, and that phage presence confers some fitness benefit leading to an overall relative fitness gain. This greater competitive fitness could have promoted its population expansion. To this end, we will first document the basic attributes of phage-bacterial interaction in Objective 1 to design experiments in Objective 2 that optimally test our specific hypothesis using competition and mixed microcosms. We will also evaluate the impact of virulence using infective models we have developed with prior Hatch support, which may also explain why phage-harboring ST36 continue to predominate clinical cases despite the ability of ST36 to be cured of these phage.Objective 1: Examine the dynamics of Vipa26 and Vipa36 phage production and impact on viable cell growth in different strain backgrounds, and determine their natural distribution, and host susceptibility range.Objective 2: Determine the impact of chronic phage infection on intra-species competitiveness, anti-predation and virulence of host strains.