Page 65 - World Aquaculture Magazine - March 2011

World Aquaculture 63
nostimulants or immunonutrients, which augment the im-
mune system of cultured aquatic animals. Immunomodula-
tion by dietary manipulation may, therefore, offer a novel
alternative to reduce the use of antibiotics and other drugs
in aquaculture, which in turn decreases the residual load of
those compounds in the food chain as well as in the aquatic
environment. Some researchers have reported increased fish
survival by using immunonutrients (Kawakami et al. 1998).
There are several polymers of carbohydrate and their
derivatives are being used as immunostimulants. Among
the most popular and effective immunostimulants is
β-glucan, found in the cell walls of plants, fungi and bac-
teria. Fructose-based polymers are also categorized as
the bioactive polysaccharides. Microbial levan is shown
to have immunostimulating properties. Dina et al. (2007)
studied the effects of dietary levan on the survival of Cy-
prinus carpio juveniles fed a feed containing levan at con-
centrations ranging from 0.1 to 1.0 percent. One hundred
percent survival was obtained with 0.5 percent levan in the
feed. Increasing the concentration to 1 percent probably
increased the antigenic load, leading to immunosuppres-
sion and thus, reducing the protection efficiency. This is
in agreement with Anderson (1992) who proposed that an
inadequate amount of immunostimulant will result in no
protection, whereas too much may cause immunosuppres-
sion. Levan activates the non-specific phagocytes, which is
important for reducing mortality in fish.
Recently, Gupta et al. (2008) reported that levan at 1.25
percent can be used as a dietary immunostimulant for
Labeo rohita juveniles. According to him, hemoglobin con-
tent and total leucocyte count were increased with dietary
supplementation of levan at 1 percent or more. An increas-
ing trend for total erythrocyte count was observed with in-
creasing levels of dietary levan. Lower levan-supplemented
groups showed a higher albumin/globulin ratio. As levan
supplementation was increased, there was a gradual increase
in serum lysozyme activity and respiratory burst activity
[nitroblue tetrazolium (NBT) assay] reduction values. The
highest lysozyme activity and NBT were observed in the diet
supplemented with 1.25 percent levan. No significant histo-
architectural changes were associated with dietary levan lev-
els. The relative survival percentage of juveniles after chal-
lenge with Aermonas hydrophila was the highest in the 1.25
percent fed group. These studies have been the only reports
submitted presenting data on levan application in aquacul-
ture, which should be explored in more detail with various
species of fish and shellfish.
Other applications of levan
In addition to the use of levan in foods, feeds, medicines,
and cosmetics, other rare uses of levan have been reported by
the chemical and biotech industries. Levan has been shown
to exert excellent cell proliferating, skin moisturizing and
skin irritation-alleviating effects as a blending component in
cosmetics. Levan derivatives, such as sulphated, phosphated
or acetylated levans, are asserted to be anti-AIDS agents.
In addition, levan is used as a coating material in a drug
delivery formulation. Levan has a number of industrial ap-
plications, such as a surfactant for household use and a gly-
col/levan aqueous two-phase system for the partitioning of
proteins. However, there are some limitations for the indus-
trial applications of levan as a result of its weak chemical
stability in solution and the complex process required for its
purification. Once those limitations are resolved, the market
for levan will gradually increase.
Future Prospects
To date, only a meager amount of information is avail-
able about the immunostimulatory efficacy of levan in the
diet. Further feeding trials are necessary before advocat-
ing the use of levan under immunosuppressive or stressful
conditions associated with intensive culture practices. The
immunostimulating effect of levan in L. rohita fingerlings
was found to be higher at the 1.25 percent dietary level but
the optimum was 0.5 percent for Cyprinus carpio juveniles.
However, the effect of a higher inclusion level on immunity
is not known and needs further study. The route of admin-
istration of levan may also affect immunity as reported in
several studies in the case of glucan. Hence, both route of
administration along with dosage may boost the immune
system. It appears that microbial levan is a potential immu-
nostimulant in aqua-feed but the dose depends on the spe-
cies and the type of management practices.
Conclusion
The research on prebiotics and immunonutrients is
mounting with the ever-increasing need for environmentally
friendly aquaculture. Even though some information on the
use of microbial levan as a prebiotic and immnonutrient has
been documented, it is limited to a few species. More precise
research information on the application of microbial levan
could provide an eye opening direction for betterment of
the aquaculture industry. In this article, we have tried to in-
corporate some valuable information on the present status
and future prospects of microbial levan and its use in aqua-
culture.
Notes
1Central Institute of fisheries Education, Seven Bungalow, Versova,
Mumbai-61, India
*Corresponding author:pronobjaan80@gmail.com.Phone:
+919769733660, Fax: +912226361573
References
Anderson, D. P. 1992. Immunostimulants, adjuvants, and vaccine
carriers in fish: Applications to aquaculture. Annual Review of
Fish Diseases 2:281-307.
Bello, F. D., J. Walter, C. Hertel and W. P. Hammes. 2001. In vitro
study of probiotic properties of levan-type exopolysaccharides
from Lactobacilli and non-digestible carbohydrates using de-
naturing gradient gel electrophoresis. Systematic and Applied
Microbiology 24:232-237.
Dedonder, R. 1966. Levan sucrase from Bacillus subtilis. Methods
in Enzymology 8:500-505.
Dina, R., A.K. Pal, Z.P. Bhathena, N.P. Sahu, A. Jha and S.C.
Mukherjee. 2007. Dietary Microbial Levan Enhances Cellular
Non-specific Immunity and Survival of Common Carp (Cypri-
nus carpio) Juveniles. Fish & Shellfish Immunology 22:477-486.
(Continued on page 66)