Bacteriophages improve survival and metamorphosis of larval Pacific oysters (Crassostrea gigas) exposed to Vibrio coralliilyticus strain RE98

The bacterial pathogen Vibrio coralliilyticus has been implicated in severe larval oyster mortality events in U.S. hatcheries. The aims of this study were to determine whether additions of bacteriophages (phages) specific to V. coralliilyticus strain RE98 can lower concentrations of V. coralliilyticus in seawater and consequently enhance survival, growth and metamorphosis of larvae of the Pacific oyster Crassostrea gigas). Cocktails containing 2 or 3 previously characterized phages in the family Myoviridae (designated 6B, 7B and 11A) were added to cultures of C. gigas larvae together with V. coralliilyticus RE98. Results confirmed that: 1) the addition of a 2-phage cocktail (7B and 11A) significantly reduced concentrations of V. coralliilyticus in autoclaved seawater over a 48-h period in the absence of larvae (p < 0.05); 2) the concentration of a 2-phage cocktail required to significantly reduce mortality of 2-day post fertilization (PF) larvae depended on the initial concentration of V. coralliilyticus added to the seawater; 3) preparation of three-phage cocktails using tangential flow filtration increased the effectiveness of the phages; and 4) a single addition of a cocktail containing 3 phages to 2-day old oyster larval cultures eliminated mortality due to exposure to V. coralliilyticus and significantly improved the proportion of larvae metamorphosing to become spat more than 16 days later. Reductions in V. coralliilyticus-associated larval mortalities by additions of phages were consistent across a range of culture conditions, varying from 1 ml cultures with autoclaved seawater to 10-l cultures supplied with non-sterile, 10-μm filtered seawater.