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Prebiotics: A pathway to achieving optimal fish health

October 2, 2023
Prebiotics: A pathway to achieving optimal fish health

Prebiotics are primarily known for their role in stimulating beneficial gut bacteria; however, this extensive group of compounds serves a multitude of functions.

  • They enhance the protective mucus barriers of the skin, gills and gut.
  • They improve gut integrity, digestion, nutrient uptake and immune function, and they help to bind and remove external pathogenic bacteria from the gut, protecting the fish’s health.
  • Consequently, they facilitate the cultivation of a diverse and healthy composition of gut bacteria, commonly known as a healthy microbiome.

Considering these benefits, the use of prebiotics aligns with a holistic approach to enhancing overall fish health and growth, reducing the need for antibiotics, and use of vaccinations.

The predominant sources of prebiotics are non-digestible carbohydrate fractions. Commercially available examples include β-glucans; oligosaccharides derived from galactose, fructose or mannose; organic acids; inulin; fructo-oligosaccharides (FOS); and many others, some of which are combinations of these elements. In this article, our primary focus will be on understanding the mode of action associated with prebiotics and the impact they have on the immune and digestive systems. More specifically, we’ll focus on mannan oligosaccharide (MOS), since it has been studied extensively across different fish species. This compound has demonstrated its efficacy under practical conditions on fish farms, displaying support for both fish growth and overall health across varying environmental contexts.

Three lines of defence protecting fish health

The immune system consists of an extensive network of diverse cells and tissues which work together to protect the vital functions of the body from external pathogens. These potential threats primarily include bacteria, viruses, parasites and fungi. They cause infections in an attempt to establish themselves within the host. The risk is heightened in aquaculture, because fish lack the ability to migrate to optimal environmental conditions. This increases their susceptibility to disease expression.

The immune system can be subdivided into three lines of defense against the various forms of pathogens.

First line of defense: blocking pathogens and foreign materials from entering the body

A protective mucus layer covers the entire surface area of physical barriers, such as the skin, gills and gut. Diverse microbiota optimize health through the competitive exclusion of bacteria, which reduces the risk of pathogens taking hold of the microbiome.

Fig. 1: The first line of defense prevents pathogens from entering the body by mucus layers and cell barriers (from left to right, the protective tissue and mucus layer of the skin, gills and gut)

Second line of defense: the innate immune system

The activation of the innate immune system takes place when pathogens successfully pass through external barriers and attempt to infect the organism. The innate immune system attacks invading pathogens using white blood cells (leukocytes), which can differentiate between “self” and “non-self” cells. It targets cells that lack the recognizable marker molecules of the body. Within this system, immune activity is further regulated by inflammation and antimicrobial proteins.  

Third line of defense: the adaptive immune system

The adaptive immune system differentiates itself from the innate immune system by stimulating a pathogen-specific immune response. It singles out and eradicates a single pathogen, earning the title “specific immune system.” This response is also known as the secondary immune response and forms the foundation for vaccinating many animal species.   

A diagram of different types of resistance

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Image 1: Overview of the immune system

The digestive tract: facilitating nutrient uptake and immune function

Expertly formulated prebiotics in aquafeed can optimize immune defense, resulting in significant alterations within the digestive tract. They particularly affect the structure of the gut and the composition of the microbiome.

After ingestion, macronutrients such as proteins, fats and carbohydrates undergo a series of digestive processes that break them down in preparation for absorption and assimilation by the fish. These smaller components enter the body through the gut wall, which is lined with microvilli, structures that increase the gut’s surface area, promoting increased nutrient absorption.

A compromised digestive tract can lead to poor performance, characterized by a higher feed conversion ratio (FCR) and reduced immune response against pathogens. Facilitating a diverse microflora population is essential for enhancing intestinal development, ensuring gut integrity and optimizing the digestion process. Within the gut, mucus-producing cells promote a thick and protective mucus layer, protecting the delicate tissue underneath.

Mode of action of MOS for different fish species

Now that we have a thorough understanding of the immune system and digestive tract, let’s delve deeper into examining the specific impacts of the prebiotic mannan oligosaccharide (MOS). MOS is derived from the yeast Saccharomyces cerevisiae, commonly known as “baker’s yeast.” Through a sophisticated refinery process, MOS is extracted from the yeast and incorporated into the feed ingredient mixture. The effectiveness of MOS is determined by several factors, including fermentation conditions, genetic strains and various processing parameters. As a result, not all forms of MOS yield identical effects. At Alltech Coppens, we combine Bio-Mos® and Actigen® with a chelated mineral mix, Bioplex®, to create Aquate®, which is integrated into all our feed formulations.

A group of fish with text

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Image 2: Results after Aquate technologies were adopted in the Mediterranean Sea bass industry on large-scale commercial farms

At Alltech, research is the primary focus. To determine the effects of Bio-Mos on fish performance and gut health, several R&D actions were undertaken.

For rainbow trout, it was observed that beneficial bacteria colonization was promoted in the gut of a healthy individual when gut bacterial load was reduced (Dimitroglou et al. 2007). Furthermore, Bio-Mos has demonstrated improvements in microvilli density and length, contributing significantly to improved nutrient absorption and enhancing fish performance (Sweetman et al. 2008). Finally, Bio-Mos can enhance the thickness of the mucus layer across the skin, gills and gut, creating a prophylactic effect for many fish species (Sweetman et al., 2010).

The effects of MOS on fish health and growth have been extensively documented across numerous peer-reviewed papers. In rainbow trout, the inclusion of MOS positively influenced growth rates and improved FCR and survival rates. It also displayed positive effects on growth in a wide variety of other species, such as brook trout, sturgeon, common carp, koi, African catfish, European sea bass and sea bream. Additionally, beneficial effects were observed in gut structure, pathogen-binding capacity (both in vitro and in vivo), immunostimulant properties, and nutrient digestibility for many fish species (Ringø et al. 2014).

Extensive research on sea bass (Dicentrarchus labrax) demonstrated a more robust immune system and improved resistance to Vibrio alginolyticus. In a separate sea bass study, notable improvements in intestinal tissue structure, increased stress resistance, and improved mucus production were noted, all contributing to strengthening protective barriers. Sea bream displayed positive outcomes in terms of protein digestibility, intestinal tissue enhancement and the modulation of the microbiome (Ringø et al. 2014).

The benefits stemming from the inclusion of prebiotics in aquafeed play an important role in fostering sustainable and efficient growth as well as promoting the health of fish. To delve deeper into the Alltech Gut Health program, watch this explainer video or visit



Dimitroglou, A., S. Davies, R. Moate, P. Spring and J. Sweetman (2007). The beneficial effect of Bio-Mos on gut integrity and enhancement of fish health. Presented at Alltech’s Technical Seminar Series held in Dublin, November 2007.

Sweetman, J., A, Dimitroglou, S. Davies, and S. Torrecillas (2008). Gut morphology: a key to efficient nutrition. International AquaFeed, 11, 27-30.

Sweetman, J. W., S. Torrecillas, A. Dimitroglou, S. Rider, S.J. Davies and M.S. Izquierdo (2010). Enhancing the natural defences and barrier protection of aquaculture species. Aquaculture Research, 41(3), 345-55.

Ringø, E., A. Dimitroglou, S.H. Hoseinifar and S.J. Davies (2014). Prebiotics in finfish: an update. Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics, 360-400.

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