Dietary supplementation of commensal Cetobacterium somerae ameliorates the problems associated with fish meal replacement by plant proteins in fish
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- Institutt for biologi 
Aquaculture is growing at high speed and the expansion of aquaculture production has been accompanied by the rapid increase in aquafeed production. Fish meal (FM) has been traditionally the first choice of protein source in aquafeeds due to its high protein content, excellent amino acid proﬁle, high nutrient digestibility, and general lack of antinutrients. However, the production of FM has reached its maximum level over the last decade and the price is increasing due to the increased demand. The unbalance between the FM supply and demand has stimulated the exploration of dietary alternative protein sources. Among alternative protein sources, plant proteins are some of the most promising because of the security of supply, low price, and amino acid composition. However, most plant proteins are known to contain a wide variety of anti-nutritional factors, which can affect the utilization of proteins, minerals, or vitamins. Plant proteins can be processed to reduce the content of anti-nutritional factors before being used in the aquafeed. Ultra-micro ground mixed plant protein (uPP), which was made by mixing high-quality protein sources, was used in the study. In addition to the routine procedure of heating and solvent extraction, uPP was subjected to micronization by ultra-micro grinding (< 200 μm), which was expected to further decrease the negative effect of anti-nutritional factors. And feeding uPP to piglets and poultry can improve their growth and health. However, there is little information about its suitability in fish feeds. Probiotics are reported to degrade anti-nutritional factors. Some research proved that dietary plant proteins supplemented with probiotics can increase fish growth and immunity. Although there were several examples where exogenous probiotics proved beneficial for fish, many studies using exogenous probiotics were inconclusive or showed no beneficial effect on fish. Commensal bacteria of the host can be an alternative probiotic source within aquaculture as their beneficial characteristics such as growth promotion, increase of survival, enhancement of immunity, and resistance to pathogens. Therefore, the goal of the research work was to investigate the levels of uPP replacing FM and seek commensal bacterium that may mitigate the negative effects associated with the addition of uPP. Study the effects of commensal bacterium on the health of fish that have been fed diets containing uPP. Evaluate whether the commensal bacterium can be used as a probiotic to improve fish health. It was concluded that dietary 2.5%uPP can be adopted to further improve FM replacement, whereas dietary 5%uPP impaired the gut and liver health of common carp. Notably, the addition of 5%uPP negatively changed the gut microbiota, significantly reducing the relative abundance of commensal bacteria Cetobacterium. The addition of Cetobacterium somerae reversed the negative impact of 5%uPP inclusion on fish health, indicating that it can be used as a feed additive to improve the percentage of FM replacement by uPP. Furthermore, the addition of C. somerae improved liver and gut health, as well as antiviral immunity, further suggesting that C. somerae can be used as a probiotic to improve fish health.