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.
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?
ReplyDeleteOne 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.
DeleteWhilst 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.
ReplyDeleteI 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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