Combating diseases in fatty fish with probiotics

Studies have shown that using probiotics to alter gut microbes in order to control diseases in fish have provided positive outcomes. Using probiotics such as Lactobacillus and Bifidobacterium to control diseases that disturb lipid metabolism has become a recent research area, as the gut microbiota is tightly linked with influencing the host metabolism, energy and lipid metabolism, fat distribution, insulin sensitivity and growth performance. The addition of probiotics could therefore prevent such diseases linked with lipid metabolism by decreasing serum lipid content and improving other health factors of fish. Fish use triglycerides (TAG) and cholesterol from their diet as a main source of energy, however in order to be absorbed, they must be resynthesized via two main processes – the glycerol phosphate pathway (which occurs mainly in the liver and adipose tissues) and the monoacylglycerol pathway (which takes place in the intestine).

This study by Falcinelli et al. looks at the effects of supplementation of the probiotic Lactobacillus rhamnosus on the gut microbial community and lipid metabolism of Zebrafish (Danio rerio). By coupling high-throughput sequencing with biochemical, molecular and morphological analysis, the effect of L. rhamnosus on intestinal epithelial structures, total body cholesterol, TAG and non-polar fatty acids and zebrafish larval growth was investigated. Genes involved in lipid metabolism such as those that regulated lipid synthesis, traffic storage, and homeostasis, were used.  

Results showed that there was a significant change in the microbiota community in the Zebrafish digestive tract which was due to the probiotic supplementation. These changes varied the expression of a network of genes involves in the physiological control of lipogenesis, lowered total body cholesterol and triglycerides, increased non-polar fatty acids, improved intestinal epithelium structures (i.e. microvilli and enterocytes) and reduced lipid droplets as well as increasing growth of probiotic treated larvae. The addition of L. rhamnosus reduced the presence of bacteria that contain potential pathogens, along with increasing other lactic acid bacteria. Increased microvilli and enterocyte height indicate the addition of L. rhamnosus expanded the intestinal structures, allowing better gut function and overall health. The decrease in genes agpat4 and dgat2 (both involved in the synthesis pathways of TAG) gave evidence of reduced TAG levels in larvae. TAG together with sterol esters form lipid droplets, which the gene fit2 has a correspondence with. The knock-out of this gene led to a decrease of lipid droplets in the liver and intestine. There was also an up-regulation in the cck gene expression (coding for gallbladder contraction and pancreatic enzyme secretion) with supplementation which led to increased bile production and therefore better breakdowns of lipids.

Overall this paper provides in-depth results of how the addition of a particular probiotic bacteria can cause many positive changes to the lipid metabolism of fish through gut microbiota community alterations, as well as enhancing growth and morphological features. The method involves using state of the art technology such as high-performance liquid chromatography for biochemical analysis and real-time PCR for genetic analysis to provide results only specific to the aim. These methods could be used not only on model species for biomedical research, but also commercially important marine species which require knowledge on preventing the spread of diseases in aquaculture systems.

Reference:

Falcinelli, S. et al., 2015. Lactobacillus rhamnosus lowers zebrafish lipid content by changing gut microbiota and host transcription of genes involved in lipid metabolism. Scientific Reports, 5(9336), pp. 1-11.

http://www.nature.com/articles/srep09336