Thursday 13 February 2014

Probiotic Strains for Shellfish Aquaculture: Protection of Eastern Oyster, Crassostrea virginica, Larvae and Juveniles Againsl Bacterial Challenge.

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.

4 comments:

  1. Have the authors of this study investigated what effects of probiotics had directly on the bacterial pathogens causing the mortality? I wonder if they investigated in more detail the effect of the oysters microbiota, and which harmful bacteria associated with the oysters were affected, for instance by identifying the bacterial communities before and after the treatment?

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    1. One of the two probiotics was isolated from an oyster shell, the other from sediment. They didn't investigate actual mechanisms but growth inhibition as the deciding factor in the competition assays. I suppose an interesting approach would be just to simply plate up the microbiota before and at the end of probiotic exposure to see what groups were dominant. You might assume that the probiotics would be at the end but perhaps they promote growth of competitors of the pathogen. It also wouldn't account for non culturable organisms which could be playing an important role. Hope that answers your question.

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  2. Whilst probiotics may be comparatively environmentally friendly, are they really that great for the environment? They may confer a health benefit to the target host, the oysters, though if they need to be administered often, as you suggest, their concentrations in the surrounding environment will be quite substantial and may have deleterious effects. Could the environment influence/modify the administered strain to one less beneficial and potentially more harmful? Inputting large quantities of bacteria (despite it being labelled 'good bacteria') surely can't be a good thing, though I agree that an alternative to antibiotics is necessary.

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    1. I don't think it's that they are more environmentally friendly as this is pretty much unknown, it's more that we need a replacement for antibiotics before we force a world where antibiotic resistance is the norm, where naturally occurring pathogens already have resistance before they are even introduced to the human body (not sure if there is crossing over of medical and aqua-cultural use of antibiotics, but still a worry). Yes this needs to be further investigated before implemented but then comparatively speaking, we put wind farms in the sea without fully understanding the impacts because we couldn't afford (not only monetarily speaking) to keep burning coal/oil/gas for power. People in charge will go for what makes them money and gives good publicity. I can't imagine how you would go about testing the impacts of probiotics either, as there will be/are different microbes for each organism/pathogen and billions of environmental microbiota to test interactions with. Perhaps something like this would lead to a flourishing natural oyster population? The only real solution to aquaculture is to have completely closed systems where the waste is managed and contained separate from the environment. This is however the most expensive form and much of the worlds most intensive aquaculture is developing regions where keeping cost low is the top priority. Horizontal gene transfer in the environment could be a problem but it is ongoing anyway, can;t think of any way of stopping it but if you are constantly supplying fresh probiotics at least this wouldn't affect your farmed organisms.

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