Reducing antibiotic use in marine larviculture by probiotics

Aquaculture is the fastest growing agricultural industry providing healthy food for mankind. In addition,culture of high valued marine fish, crustacean and mollusk species is financially attractive. However,diseases at the larval stages constitute a major bottleneck and cause economic losses to the industry.Vaccines are not effective at the larval stages and antibiotics are used for disease control, although thereare serious concerns about development of bacterial antibiotic resistance and its transfer to humanpathogenic bacteria. There is a strong need for development of non-antibiotic disease control strategies,especially at the larval stages.The objective of our work is to reduce the need for antibiotics in marine larviculture by developingprobiotic strategies; probiotics being defined by WHO as “live microbial cultures that excert a beneficialeffect on the host”. Rearing of marine larvae is difficult, as the larvae require live feed (Artemia, rotifers,copepods) which also requires live feed (algae). Larval pathogens can be introduced from the live feed andwe work on applying our probiont strategy for pathogen control not only at the larval stage, but also in livefeed cultures, hence targeting pathogen control at the very beginning of the production chain in aprophylactic strategy.We collaborate with several aquaculture industries rearing turbot, sea-bass, sea-bream, oysters andflounder. We have at these sites isolated bacteria that are capable of antagonising fish larvae pathogensand that are not detrimental to the fish larvae. At all sites, bacteria belonging to the marine Roseobactercladehave been isolated as strong pathogen-antagonising bacteria. We have demonstrated that thesebacteria can antagonise pathogens (Vibrio anguillarum and Vibrio harveyi) in live feeds (algae, rotifers,Artemia) and that they have a dramatic and significant disease-reducing effect in turbot and cod larvaechallenged with pathogenic Vibrio.We are elucidating the mechanisms by which the probionts exert their effect, and have by mutagenesisidentified tropodithietic acid (TDA) as an important molecule in the pathogen-antagonism. However, othermolecules and mechanisms are likely also involved. Understanding the spectrum of mechanisms of action isimportant to determine where and how the probionts should be applied and also in determining potentialside effects that could arise for the probiotic bacteria.Other studies have focused on fish pathogens and it has been suggested that introducing lactic acidbacteria that are used as human probiotics (and have GRAS status) could be a way forward. However, webelieve that re-introducing (or boosting) a potential probiotic bacterium already present in the fish larvaefeed and rearing environments is likely a more successful strategy than introducing a foreign bacterium to this environment. Indeed, our results on disease prevention in model systems have been very convincing.