3. Prebiotics

3.1. Definition and Application of Prebiotics

Prebiotics are indigestible food components that confer health benefits to the host by promoting the growth and activity of beneficial bacteria in the colon, ultimately enhancing overall well‐being [6]. Many food oligosaccharides and polysaccharides are recognized for their prebiotic properties; however, “prebiotics” and “dietary fibers” are often used interchangeably, leading to confusion. It is important to note that not all carbohydrates in our diet qualify as prebiotics. The necessity of rigorously defining a food component as a prebiotic is underscored by researchers who emphasize that scientific evidence must demonstrate the component’s ability to withstand gastric acidity, enzymatic breakdown, intestinal digestion, and microbial fermentation [119]. Mechanistically, prebiotics function by serving as substrates for beneficial gut bacteria, specifically fostering their growth. The fermentation of prebiotics produces SCFAs, such as acetate, propionate, and butyrate, which offer a multitude of health benefits. These include lowering intestinal pH, providing energy to intestinal cells, and enhancing gut health by reducing susceptibility to infections. Furthermore, prebiotics improve nutrient digestion and absorption by increasing enzymatic activity and facilitating the uptake of essential minerals, including calcium, magnesium, and phosphorus. Prebiotics also play a crucial role in strengthening the intestinal barrier by modulating tight junction proteins, thereby reducing gut permeability and preventing the entry of pathogens and toxins into the bloodstream.

3.2. The Selection of Prebiotics

To recognize and authenticate a compound as a prospective prebiotic, it is essential to elucidate its source, origin, purity, chemical composition, and structural traits. Prebiotics must adhere to all relevant national safety criteria and achieve a classification of Generally Recognized As Safe (GRAS) to be approved. They should also undergo thorough assessments for appropriate dosage and potential side effects, be free from contaminants and impurities, and not adversely affect the host’s intestinal microbiota [26]. It is essential to emphasize that the term “prebiotic” should only be applied when modifications to a specific microbiota result in a positive health impact. Food ingredients can be classified as prebiotics based on several key characteristics [120]. First, prebiotics must be resistant to stomach acidity, GIT absorption, and the hydrolytic action of digestive enzymes. Second, they should reach the large intestine, where they are selectively fermented by beneficial bacteria. This fermentation process can lead to modifications in metabolic pathways and enhanced immune function, ultimately contributing to the host’s overall health. A third critical factor is the ability of prebiotics to promote the growth or activity of gut bacteria that support the host’s well‐being. Finally, the technological properties of prebiotics are vital for their effective production and availability for metabolism by intestinal bacteria. By understanding and adhering to these criteria, we can better harness the potential of prebiotics to improve health outcomes (Figure 5).

Figure 5: Criteria for selecting suitable prebiotics.

3.3. Types of Prebiotics

Prebiotics are naturally found in cereals, fruits, and legumes [121]. However, commercial chemical and enzymatic processes produce numerous comparable compounds. Inulin, isomaltooligosaccharides (IMO), lactylol, lactosucrose, lactulose, mannan‐oligosaccharides (MOS), oligofructose, transgalactooligosaccharides, arabinoxylan‐oligosaccharides (AXOS), β‐glucans, stachyose, fructo‐oligosaccharides (FOS), galacto‐oligosaccharides (GOS), and xylooligosaccharides (XOS) are among the non‐digestible carbohydrates that prebiotics have been categorized [122]. However, prebiotics differ from carbohydrates, peptides, proteins, and lipids [26]. This list of prebiotic supplements is continually expanding due to the diverse range of species that can be employed to obtain them [123].

A novel class of potential prebiotics is represented by AXOS. These are fragmentation products of arabinoxylans (AX), which are found in the cell walls of many cereal grains. They comprise an O‐2 and/or O‐3‐L‐arabinofuranosyl unit linked to a major chain of beta‐1,4‐linked D‐xylopyranosyl units [124]. The intestinal flora utilizes AXOS as part of dietary fibers, with beneficial effects on various physiological processes. AXOS enhanced the Siberian sturgeon’s immunological responses and growth performance (Acipenser baerii) [125]. One of the most widely utilized prebiotics for fish is MOS, which is a glucomannoprotein complex derived from the cell wall of yeast (Saccharomyces cerevisiae). Among the several prebiotics, MOS is a significant player, as it has been shown to inhibit the adherence of certain bacteria to the gut and have positive effects on animal growth, intestinal immunity and structure, and gut microbiota [126]. Research indicates that MOS supplementation boosted the body length and weight gain of juvenile tilapia [127]. Natural polysaccharides comprising D‐glucose monomers connected by β‐glycosidic linkages are called β‐glucans. They are structural elements of bacterial, fungal, algal, and plant cell walls and serve as energy storage [128]. The term “glucan” refers to a class of carbohydrate polymers categorized as either α‐ or β‐linked according to their interchain connections. The fundamental structures and conformations of β‐glucans derived from various sources are frequently different. While the conformation of β‐glucans frequently manifests as a random coil, single helix, or triple helix and is influenced by the primary structure, intermolecular force, temperature, and solvent, the primary structure is determined by the type of glycosidic bond as well as the degrees of branching and polymerization.

Several studies have been conducted on the effects of β‐glucan, a well‐known immunomodulator that can be administered to vertebrates by injection, bath therapy, or as food [129]. When given as a prebiotic, β‐glucan increased juvenile red sea bream Pagrus major’s immunological responses, digestibility, and feed efficiency [130]. A naturally occurring prebiotic, XOS, is made up of indigestible sugar oligomers based on xylose that are joined by β1‐4 glycosidic linkages. These compounds can be broken down by the lower GIT system into SCFAs, which enhance fish species’ immunological responses and encourage the growth of advantageous Bifidobacteria and lactobacilli [131].

3.4. Prebiotic Dosage Use

Prebiotics can profoundly impact the bacterial community within the GIT, promote growth rates, and enhance the immune system’s function. Numerous researchers have dedicated their efforts to optimizing the incorporation of prebiotics into animal feed to enhance the growth rates and overall survival. Table 2 presents several key findings from various prebiotic studies, highlighting their significance and potential applications.

Prebiotics | Species | Region | Duration | Dosage | Effects | References
MOS | Nile tilapia (O. niloticus) | Zagazig University, Egypt | 12 weeks | 0.5% | Mortality rates of fish under A. hydrophila infection challenge were reduced | [49]
Galactomannan oligosaccharides (GMOS) | European sea bass (Dicentrarchus Labrax) | University of Las Palmas de Gran Canaria, Spain | 63 days | 0.5% | Increased the relative abundance of Bacteroidales, Lactobacillales, and Clostridiales | [132]
β‐Glucans | Nile tilapia (O. niloticus L., 1758) | Çukurova University, Turkey | 2 weeks |  | Increases leukocyte (WBC) and phagocytic activity levels | [133]
GOS and XOS | Tilapia (O. niloticus) | East China Normal University, China | 8 weeks | 10 g kg−1 | GOS‐supplementation improved the amino acid composition, while XOS‐supplementation showed beneficial effects on growth performance | [134]
FOS | Asian seabass (Lates calcarifer) | Chennai, India | 45 days | 5 g kg−1 | Improving the survival rate and immunological parameters | [135]
AXOS | Siberian sturgeon (Acipenser baerii) | KU Leuven, Belgium | 4 weeks | 2% | Improved growth performance and boosted immune responses | [125]
FOS | Stellate sturgeon (Acipenser stellatus) | Gorgan province, Iran | 75 days | 1% | Improved growth performance, beneficial intestinal microbiota, and stimulated immune response | [136]
Inulin | Nile tilapia (Oreochromis niloticus) | Abbassa, Egypt | 2 months | 5 g kg−1 | Enhanced weight gain, survival, and specific growth rate | [137]

3.5. Beneficial Effects of Prebiotics

Prebiotics play a pivotal role in aquaculture, offering numerous benefits for aquatic organisms and the broader agricultural ecosystem. They contribute to the maintenance of a balanced gut microbiota, which is essential for the well‐being of cultured species and a robust immune response [138]. The competition among gut microbes for prebiotics leads to the selective colonization of beneficial bacteria, effectively displacing harmful pathogens from the gut epithelial cells. These beneficial microbes further support gut health by producing SCFAs, which play a crucial role in maintaining gut integrity. Additionally, the modulation of the immune response by gut microbes enhances the functionality of the gut barrier, providing further protection against infections.

3.6. Factors Affecting Prebiotic Effectiveness

Several factors, including dosage, type, rearing conditions, fish species, and administration methods, influence the effectiveness of prebiotics in fish diets. Dose–response studies play a vital role in identifying dietary prebiotic levels that may be toxic or harmful, as well as those that provide no significant benefits. Moreover, the optimal prebiotic dosage can vary depending on the fish species, size, prebiotic type, and rearing conditions. Therefore, conducting prebiotic dose studies is essential for establishing appropriate feeding regimens that maximize health benefits for each species, age group, and rearing environment. The ability of a fish’s gut microbiota to utilize a prebiotic is contingent upon the specific properties of that prebiotic. For instance, fish species adapted to a plant‐based diet may derive greater benefits from certain types of prebiotics, while carnivorous species may respond better to others. Additionally, prebiotics can enhance the integrity of the gut epithelial barrier, thereby reducing the entry of pathogens and inflammatory agents into the bloodstream. This fortification helps fish avoid systemic infections and lowers their risk of developing inflammatory diseases. The importance of prebiotics and probiotics in fish farming is highlighted in Figure 6.

Figure 6: Illustration of the potential benefits of incorporating probiotics and prebiotics in fish farming.

3.7. Additional Factors to Consider in Administering Probiotics and Prebiotics

Numerous probiotics and prebiotics are currently available and being utilized in various industries. While incorporating these products may result in additional costs, the potential benefits include enhanced production efficiency and reduced disease occurrence. An innovative approach involves simultaneously administering substances from both groups, known as symbiotic. The primary objective of a symbiotic is to improve the implantation and survival of live microbial supplements in the GIT [6]. Despite the limited focus on symbiotics in aquaculture, the numerous advantages of probiotics and prebiotics may prompt the development of protocols for delivering these combined substances. This could lead to significant advancements in the industry and potentially revolutionize the way these products are utilized. Additionally, probiotics and prebiotics play a crucial role in promoting a more environmentally friendly and sustainable aquaculture industry by improving water quality and reducing the need for antibiotics.