Title:

Contribution of Type III Secretion to Competitive Survival of Pathogenic Vibrio parahaemolyticus

Over the past 15 years, sea temperatures in New England coastal waters have risen, as have cases of food-poisoning from pathogenic strains of Vibrio parahaemolyticus. Recent surveillance in the Great Bay Estuary (GBE) of New Hampshire revealed a population shift to greater prevalence of V. parahaemolyticus containing Vibrio Pathogenicity Islands (VPaIs) that confer human virulence. The VPaIs of these local isolates are distinct from the one characterized in pandemic strains and encode a diagnostic trh hemolysin either alone or in combination with the tdh hemolysin and a dedicated Type III Secretion System (T3SS) for delivery of a unique array of toxins. In estuarine ecosystems like the GBE, protists are significant consumers of bacteria, and a previous study showed the VPaI type found in pandemic strains confers resistance to predation by protists endemic to the Baltic Sea 1. We hypothesize that T3SS toxin delivery by local pathogenic V. parahaemolyticus confers a competitive advantage under native protist predation. To test this, we constructed microcosms of GBE water either with or without native bacteria, protist, and particles >0.22 µm and evaluated the survival and growth of V. parahaemolyticus either with or without functional T3SSs (ΔvscN1/N2). Concentrations of inoculated V. parahaemolyticus were enumerated daily by culture and flow-cytometry. In filtered microcosms, V. parahaemolyticus increased over 4 days, whereas in unfiltered GBE water they declined, suggesting predators do target V. parahaemolyticus even in the presence of other bacteria. However, the mutant strain defective for toxin secretion declined more precipitously than the parental strain. Using a single predator (Acanthamoeba castellanii) and prey laboratory model, presence of a functioning T3SS also improved survival, as with the pandemic strain. This demonstrates that native GBE microbes are likely integral in shaping the V. parahaemolyticus population, and defense against protist predation could be a driver of the successful invasion and establishment of pathogenic V. parahaemolyticus. 1Matz, C., Nouri, B., McCarter, L., & Martinez-Urtaza, J. (2011). Acquired Type III Secretion System Determines Environmental Fitness of Epidemic Vibrio parahaemolyticus in the Interaction with Bacterivorous Protists. PLoS ONE, 6(5), e20275. https://doi.org/10.1371/journal.pone.0020275

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