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.