Beta-glucans In Aquaculture

Dharmendra Kumar Meena*., Pronob Das¹., Md. Shahbaz Akhtar²., Sekar M¹.,and Satendra Kumar¹.

*Central Inland Fisheries Research Institute, Barrackpore, Kolkata, 700 120

1. Central Institute of Fisheries Education, Mumbai, 400 061

2. Directorate of Coldwater Fisheries Research, Bhimtal, 263 136

Corresponding author: Dharmendra Kumar Meena

* Email : dkmeenafnb@gmail.com

Beta-glucans are naturally occurring linear unbranched homopolysaccharide that contain only glucose as structural components, which are linked with glycosidic bonds. In nature Beta-glucans are wide spread and the most common sources are derived from the cell wall of baker's yeast Saccharomyces cerevisiae and Echinaceae members. Beta (1,3) and Beta (1,4) glucans are extracted from the bran of some grains such as oats and barley, and in small degree from rye and wheat. Other sources include some types of seaweed and various species of mushrooms such as Shiitake, and Maitake. They are constituents of some of the pathogenic bacteria (Pneumocystis carinii, Cryptococcus Neoformans) and members of Rhizobiaceae and fungi (Agaricus subrufesuns). There are soluble and insoluble Beta-glucans and research has shown that insoluble (1,3/1,6) Beta-glucan, has greater biological activity than that of its soluble (1,3/1,4) Beta-glucan18. Basically there are two types of glucan molecules a-glucan dextran with 1,6; starch with a (1,4) and a (1,6) glycosidic bonds) and Beta-glucan (cellulose with Beta (1,4); zymosan with Beta (1,3); laminarin with Beta (1,3) and Beta (1,6), lichenin with Beta (1,3) and Beta (1,4) glycosidic bond). Beta-glucans are known as "biological response modifiers" because of their ability to activate the immune system 16. They make the immune system work better without becoming overactive, lower elevated levels of LDL cholesterol, help in wound healing, prevent infections, enhance resistance against infectious pathogens and act as an adjuvant in the treatment of cancer in human, fish and animals. Beta-glucan is not an essential nutrient for human or fishes so there are no deficiency disease that may occur. Beta=-glucans are recognized as safe and non-toxic with no side effects and if so rare, occasionally an allergic reaction has been reported in human. Commercially they are available under different names such as Macrogard, Leucogard, EcoActiva used for fish feed supplementation, yeast glucan Glucasan used as food supplements for human were as Glucaferm used in face and skin creams. The dosage form is available as capsules, topical cream, and injectables, were as in case of fishes the dosage is mixed with the feed.

Structure of Beta-glucans

Glucans are a heterogeneous group of glucose polymers, consisting of a backbone of Beta (1,3) linked Beta-D-glucopyranosyl units with Beta (1,6)linked side chains of varying distribution and length. Beta-Glucans derived from different sources have some differences in their structure. Oat and barley Beta-glucans are linear with Beta (1,4) and Beta (1,3) linkages. Mushrooms Beta-glucans have short Beta (1,6) linked branches from Beta (1,3) backbone. Yeast Beta-glucans have Beta (1,6) branches further with additional Beta (1,3) regions². The Beta (1,3) linked glucan tends to form long chain that winds around themselves resembling a helix were as the Beta 1,4 linked glucan is a linear molecule like a lying out a chain, flat and straight. These structural differences can trigger difficulties in there extraction and on their activity such as larger molecular weight glucans activate leukocytes, stimulating their phagocytic, cytotoxic, and antimicrobial activities, and production of ROS. Low molecular weight glucans have less cellular effects been as very short glucans are considered to be inactive². The differences between Beta-glucan linkages and chemical structure are significant in regards to solubility, mode of action, and overall biological activity.

Role of Beta-glucans in aquaculture

Immunostimulants are chemical compounds that activate white blood cells (WBC) and may deliver fish and animals more resistant to infections pathogens. Immunostimulants have been used as feed additives from past many years in aquaculture sector and imagining modern aquaculture with out the use of injectable vaccines against different pathogens causing diseases is impossible. Beta-glucans is an important immunostimulant among which yeast Beta-glucan has been widely used in aquaculture sector. They are found to be highly beneficial in reducing the mortality due to opportunistic pathogens in juvenile fish also, in preventing viral diseases, in enhancing disease resistance to parasites and farmed shrimp, in increasing the efficacy of anti-microbial substances and vaccines¹⁹.

Dietary feed supplementation with both natural and commercial Beta-glucans has shown improvement in health, growth and general performance of different animal groups, including farmed shrimp, fish and other land animals. Fish growth has been enhanced with the use of (EcoActiva) a commercially available Beta-glucan orally in Pink snapper⁴; Beta-glucan in Rohu¹⁵; Beta (1,3) glucan in Large yellow croaker¹. Beta (1,3) glucan enhanced the protection in case of yellow croaker against Vibrio harveyi¹, Beta (1,3) yeast glucan against Aeromonas hydrophila in Asian catfish¹⁴. Beta (1,3) glucan from Saccharomyces cersvisiae along with bacterial LPS (Lipopolysaccrides) induced resistance against Aeromonas hydrophila in carp²⁷: Beta (1,3) glucan against Edwardsiella tarda in Rohu²⁴, yeast Beta (1,3) against white spot syndrome¹⁷ when injected intramuscularly and Beta-glucan from Schizophyllan against RV-PT virus¹⁰ in Kuruma Shrimp. Many Scientific experimental reports have shown that Beta-glucan as feed supplement enhance the survival rate against White Spot Syndrome in Black tiger Shrimp^8,3 and yeast Beta (1,3,) and Beta (1,6) glucans as an adjuvant in Atlantic salmon²³, Beta (1,3) glucan in Atlantic Salmon yellowtail¹² and mushroom glucan in Catla¹¹ when injected intaperitoneal. Reports with enhanced antibody response with the use of mushroom glucan in Catla¹¹, Beta (1,3) and Beta (1,6) glucan in Asian catfish¹⁴, and Beta (1,3) glucan in Rohu²⁵ are available. Effect of Beta-glucan in enhancing resistance to pathogen was studied in immunocompromised fish. Feeding Beta (1,3) glucan to aflatoxin immuno compromised rohu (Labeo Rohita) significantly enhanced non-specific immunity against Edwardisella tarad^24,25 and in Nile tilapia against S. iniae⁸. Supplementation of commercially available Yeast and yeast cell components (YYS) containing Beta-glucan in feed showed no effects on the growth, survival, resistance and protection against E. ictaluri in channel catfish in juvenile Nile tilapia against S. iniae and against E.tarda and S. iniae^26,21 respectively. Apart this reports with Beta-glucans enhancing phagocytic activity, haemagglutination, blood leucocyte number, antibod response, lysozyme and ROS production in fishes when supplemented in the feed are available.

Receptors of Beta-glucans

With the discovery of Toll- like receptors (TLR) or pattern recognition receptors immunostimulants has gained tremendous importance in aquaculture sector. These receptors bind to a wide range of pathogens. Beta-glucan receptors were first identified on the surface of monocytes with the phagocytic receptor for particulate activators of the alternative complement pathway⁵. Adaptive immunity depends on receptors that identify antigenic patterns to which the host has been exposed earlier. But innate immunity relies on genetically predetermined pattern recognition receptors (PRRs) that recognize biomolecules (carbohydrates, lipids, and proteins) specific to microorganisms and not to host²⁰. There are four different types of receptors identified to which Beta-glucan binds they are scavenger receptor (SR), complement receptor 3 (CR), Dectin-1 (βGR), Toll-like receptors (TLR2/6). They all are involved in innate response as well as adaptive response (βGR and TLR2/6)⁶. SR may bind to anionic Beta-glucans (sulphated Beta-glucans either made chemically or found in certain algae). Dectin-1 (βGR) is expressed on cells of the monocyte, neutrophils and recognizes carbohydrates containing Beta (1,3) and Beta (1,6) glucan linkages⁷. CR3 is highly expressed on neutrophils, monocytes, and NK cells and less on macrophages²⁰. It is considered as major Beta-glucan receptor. Lactosylceramide is a glycosphingolipid found on leucocytes and endothelial cells. It binds to Beta-glucans associated with production of reactive oxygen species (ROS). Toll-like receptors are used by fungal glucans and zymosan²².

Mode of action of Beta-glucans

Glucans are thought to mediate their effects via interaction with membrane receptors on macrophages, neutrophils, and NK cells. Macrophages play a critical role in all phases of host defense that is both in innate and adaptive immune responses in case of an infection when pathogen crosses an epithelial barrier; it is affected by phagocytosis of macrophages and digested by lysosomal enzymes released from them. macrophage function is determined by lysosomal enzymes and phagocytic activity. When the receptors are engaged with the Beta-glucans , the cells become more active in engulfing, killing and digesting bacteria and simultaneously they secrete signal molecules cytokines (IL-1, IL-6, IL-8, IL-12, TNF- β) which stimulate the formation of new white blood cells and other inflammatory mediators (NO and H₂O₂). Thereby activation of macrophage functions by Beta-glucans increases host immune defense. Animals with specific immune mechanisms and non-specific defence (Fish), the activated phagocytes produce cytokines molecules which can activate antibody-producing white blood cells (B and T-cells), and enhances the efficacy of the vaccines^19,13 .

General Application of Beta-glucans

Beta-glucan have various applications as an immunostimulant in human and animals. They lower coronary heart disease risk. Used in chronic fatigue syndrome, physical and emotional stress chemotherapy or radiation treatment. Used orally for colds, flu, allergies, hepatitis, lyme disease, asthma, ear infections, aging, ulcerative colitis and crohn's disease, fibromyalgia, rheumatoid arthritis, and multiple sclerosis. Beta-glucans are used for dermatitis, eczema, wrinkles, bedsores, wounds, burns, diabetic ulcers, and radiation burns. They are used for treating and reducing cancer and act an adjuvant. Used as an immunostimulant in patients with AIDS-related disorders. They are used as a food additive in products such as salad dressings, frozen desserts, sour cream, and cheese spreads. They smoothens blood sugar level and provides a low insulin response after a meal. The have cholesterol-lowering properties. They are used in cosmetics and for other skin disorders.

Conclusion

Beta-glucans have immense potential to increase the growth and survival rate in fishes when either injected orally, intramuscularly or supplemented with the feed to provide resistance to disease and protection against infectious pathogens. Beta-Glucan acts as immune system activator and cell response modifier. Dosage requirement plays an important role in providing protection against pathogen, so there is a need to design the exact dosage required to provide resistance and protection as it is dose dependent and over dosage may show adverse effect. Though different sources of Beta-glucan have been used to stimulate immunity in fishes but still Beta-glucans from chrysolaminaria, lentinan etc are yet to be explored further for aquaculture sector.

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