The bacterial species Vibrio coralliilyticus has been linked to mass mortality events of Crassostrea gigas larvae in hatcheries on the U.S. Pacific Coast. These mortality events reduce supplies of seed to farmers. In aquaculture, multiple studies report the benefits of rearing shellfish larvae with putative probiotics for protection against pathogen-induced mortalities.
In our probiotic studies, we found that additions of three marine bacteria isolates were highly beneficial to oyster larvae cultured under standard hatchery conditions. A single dose of the probiotic combination to one-day-old larvae increased subsequent larval metamorphosis success for both the Miyagi and Midori stocks of C. gigas, compared to controls with no additions of probiotics.
In axenic larval culture assays, V. coralliilyticus was found to suppress the larval immune response while also prompting cell death. Probiotic treatment prior to V. coralliilyticus infection stimulated expression of larval genes involved in immune signaling proteins and effectors. This immunomodulation in response to probiotic treatment corresponded to an increase in the survival of C. gigas larvae infected with V. coralliilyticus (Figure 1).
Further development of the axenic larval culture protocol has shown that a very small dose of V. coralliilyticus (20 CFU mL-1) results in 94% larval mortality compared with 5% mortality of the non-pathogen exposure control. Application of only 207 total CFU mL-1 of the probiotic mix reduced larval mortality to 8% when challenged with 2 × 103 CFU mL-1 V. coralliilyticus, further highlighting the effectiveness of probiotics in protecting oyster larvae against this bacterial pathogen under defined assay conditions.