Tilapia are an important species
in aquaculture because they can be fed herbivorous/omnivorous diets and can
survive in a wide range of salinities making them useful inland as well as in
coastal projects. They produce a bland tasting white meat that is desirable in
mass produced foods such as fishcakes and other frozen goods. Therefore any way
that the farming process can be improved will be beneficial. As we know, one
such technique is the incorporation of probiotics into feed. These can increase
growth or provide protection against disease. When producing new probiotics
investigations must be carried out to determine their efficiency. Probiotics with
little or no effect can be damaging relating to unnecessary waste of resources
and investment from the aquacultural industry. In particular it is important
that the target fish species is used in experiments to determine how it will
respond as extrapolation from other species is misleading and can be completely
false.
Streptococcus spp. cause streptococcosis in fish worldwide and are
of particular import to tilapia. To test the efficacy of several probiotics in
improving disease resistance, a research group (Ng et al., 2014) fed tilapia for 8 weeks on
diets containing 3 different probiotics at either 0.1 or 0.3% of the basal
tilapia diet. After this period tilapia were challenged with Streptococcus agalactiae and maintained
on their respective diets for a further 23 days. They found that in probiotics
containing Bacillus subtilis and a
mixture of Bacillus spp. and Pediococcus spp. both increased the rate
of survival at 0.1% of the basal tilapia diet. At 0.3% Bacillus subtilis and Bacillus
licheniformis did also. This shows that these probiotics are effective in
protecting tilapia from infection and are therefore valuable probiotics for use
in industry.
However there are some issues
worth addressing. Although there is now evidence that Bacillus subtilis confers protection to tilapia it should be noted
that the reason why was not fully understood. It could be that the rearing
conditions and experimental design led to a situation where the tilapia were
particularly sensitive to infection but the addition of Bacillus subtilis was enough to outcompete the pathogen within the
organism. This could mean that different rearing environments (tilapia are a
globally important species) could yield different results. In other parts of
the world the gut microbiota may be different enough for Bacillus subtilis interactions to be changed. Higher haematocrit levels
in Bacillus subtilis fed fish could
indicate that improvement in overall fish health was responsible for reductions
in disease susceptibility though further investigation would be required.
Overall this study highlights the
difficulty faced by the aquacultural industry with regards to probiotics. While
broad spectrum antibiotics are easy enough to design a probiotic that works on
the immune response, growth rate and protein content of every commercial fish
species is unlikely to ever be discovered so the slow and painful process of
testing potential microbes with all the different economically exploited fish
species seems to be the only option.
Ng, W.-K., Kim, Y.-C., Romano, N., Koh, C.-B., & Yang,
S.-Y. (2014). Effects of Dietary Probiotics on the Growth and Feeding
Efficiency of Red Hybrid Tilapia, Oreochromis sp., and Subsequent Resistance to
Streptococcus agalactiae. Journal of Applied Aquaculture, 26(1),
22–31.
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