Streptomyces never tasted so good!

In recent years, fish meal as a component to animal feed in aquaculture has become more uncertain as it has become increasingly expensive. This prompted the authors of this paper to look for cheaper alternatives to fish meal. Prior to their study, there were some reports showing that some micro organisms incorporated into animal feed resulted in promising outcomes. For example, effective protection against white spot virus syndrome (WSSV) in Black Tiger Shrimp (Penaeus monodon), the inhibition of vibrio sp. biofilm formation and vibrio sp.-caused disease, and increased growth sizes in the juvenile prawn Macrobrachium idella. The micro organisms used in these examples were Streptomyces, a marine actinobacteria, which wasn’t given much attention in the poultry or aquaculture industries. The species chosen in this experiment to consume the altered animal feed were the ornamental fish species Red Swordtails (Xiphophorus helleri).The lack of attention in this micro organism combined with the fact that it has shown proof of effectiveness in other organisms is what lead the authors into conducting this study.

Four species of marine sponge (Callyspongia diffusa, Mycale mytilorum, Tedania anhelans and Dysidea fragilis) were collected by divers, and Streptomyces were extracted, isolated and characterized. They were characterized by means of nutritional, morphological, physiological and biochemical properties. Eight different feeds were produced; 7 probiotic feeds containing Streptomyces and the control feed. The control feed used ingredients of fish meal, rice bran, groundnut oil cake and chickpea flour, using a tapioca flour binder. These were ground up and mixed with water to form a dough which was steam cooked and allowed to cool. After cooling, the dough was put through a pelletiser, and the pellets were then dried. The probiotic feed was also a dough steam cooked and cooled, but with a known quantity of Streptomyces in it. Again the dough was put through a pelletiser, and the pellets were freeze dried. The Red Swordtails were divided into two tanks, with 20 individuals in each. One tank contained the probiotic feed, and the other contained the control feed. They were fed once a day for a duration of 50 days.

After 50 days the fish were measured, showing that all fish which consumed the probiotic feed containing Streptomyces had grown larger than those who fed on the control feed. This included significantly higher absolute growth rate, specific growth rate and relative growth rate. Arriving at this result, the authors state that the use of Streptomyces in probiotic feed has a significantly positive effect on the growth of the ornamental fish. They also state that Streptomyces in probiotics will be used, and be important in aquaculture nutrition.

This paper was a good read and it is interesting to see how different bacteria could bring a variety of advantages by being incorporated into animal feed. However, there were a few points and explanations that I think this paper missed out on. When suggesting the known examples of where micro organisms have had positive effects in animal feed, they did not specify which vibrio sp.-caused disease the bacteria stopped. The authors also failed to explain why fish meal in animal feed has become more ‘uncertain’, so I assumed that it was due to the rapid price increase that was also mentioned. Finally (and this might just be me being picky), I think their statement of ‘in the near future, applications of Streptomyces as probionts will play an important role in aquaculture nutrition’ is quite a bold statement. I know it is a great find and has had a good effect on some species, but it hasn’t been used on more commercially valuable species such as salmon, so to say it will be important is a bit bold in my opinion.

Dharmaraj, S. & Dhevendaran, K. (2010). Evaluation of Streptomyces as a Probiotic Feed for the Growth of Ornamental Fish Xiphophorus helleri. Food Technology and Biotechnology. 48 (4): 497-504.