Probiotics Not Antibiotics.
Crassostrea virginica (eastern oyster) is a species of oyster of significant economic importance in the Gulf of Mexico and Atlantic coasts of North America. In recent years stocks have been plagued by infections, especially in relation to juvenile and larval mortality. This leads to significant drops in recruitment, which can be crippling to the mariculture output. Amongst these pathogens are many Vibrio spp. and Roseovarius crassotstrea, the causative agent of juvenile oyster disease. One Vibrio of importance is Vibrio tubiashii, which causes virbiosis and attacks the larval stage. Good husbandry techniques such as regular water changes can help to avoid infection though this is still high risk for the fishery owners who favour the use of antibiotics. Antibiotics not only promote the evolution of resistance in pathogens but can also have damaging environmental effects in open system mariculture, for example they may affect symbiotic microbial species in wild populations. One potential solution is the use of probiotics in place of antibiotics. These may of course have negative side effects for the environment and undoubtedly these will be uncovered, however the potential positives for aquaculture, coupled with the fact that a replacement for antibiotics must be found, make them an attractive alternative.
To test the effectiveness of some such probiotics, first potential probiotics need to be identified. The authors of this study did this through a series of bacteria-bacteria competition assays (colony-on-top assay & membrane overlay assay) with known pathogens and potential probiotics isolated from the natural habitat of the eastern oyster in Rhode Island. The most promising strains were Phaeobacter sp. S4 and Bacillus pumilus RI06-95 so these were brought forward to bacterial challenge trials involving juvenile and larval oysters (separately). Another aspect investigated was the longevity of the effects. This was done by the addition of pathogens at time intervals after removing the probiotics.
Both probiotic strains were found to significantly reduce mortality in bacterially challenged larvae and juveniles and neither had any known negative effects (mortality was tested for using controls). These benefits were achieved at a concentration that could realistically be achieved in commercial oyster cultures making the use of these probiotics feasible. The benefits, however, are not transferred from the probiotic to the organism long term so a constant supply of probiotics would be necessary. However with a short doubling time (roughly 3 hours in oyster culture medium) these may be self-sustaining depending on the flow through rate of the tank and available nutrients. One method of supply would be mixing probiotics with the feed organism, usually microalgae, though the first step towards this would be testing the effect of these strains on the feed organism(s). They also discovered that the initial screening process did not necessarily predict the in vivo effect as Bacillus pumilus did not inhibit the growth of V. tubiashi in vitro yet provided protection for both larvae and alga in vivo insinuating a symbiotic relationship or flaw in the assay process.
These findings further support the use of probiotics in aquaculture and could provide a more cost effective and environmentally friendly option over antibiotics. Keeping live cultures of probiotics on site could also help reduce the mounting costs of husbandry. Future studies should investigate what effects these probiotics have on feed organisms as well as the potential for use with other molluscan species that are affected by these pathogens.