Protection of cod larvae from vibriosis by Phaeobacter spp.: A comparison of strains and introduction times

Infections with Vibrio anguillarum and other pathogenic Vibrio spp. are a major problem for marine larviculture, and improved control of the microbiota in marine larvae cultures could ensure a more reliable and cost-effective production of juvenile fish. Phaeobacter gallaeciensis is capable of reducing V. anguillarum in live feed cultures and can, in challenge trials, protect fish larvae from vibriosis. The purpose of the present study was to estimate the probiotic potential of Phaeobacter isolates that produce different levels of the antagonistic compound tropodithietic acid (TDA). We compared the capability of three wild type Phaeobacter strains to reduce cod larvae mortalities in challenge trials with single cod (Gadus morhua) embryo/larvae cultures, and assessed the importance of the time point at which the probiotic bacteria were introduced relative to the pathogen. All three Phaeobacter strains reduced larvae mortalities, however to different degrees. The capability of the strains to prevent disease was correlated with their in vitro TDA production, as measured by in vitro inhibition of V. anguillarum. The most effective time to apply the probiotics was in advance of the pathogen, while simultaneous introduction was only effective for the two strains with the highest TDA production. This suggests that prophylactic use of Phaeobacter spp., where the probiotic bacterium is introduced early into the system, is most efficient in disease prevention.