Page 63 - World Aquaculture Magazine - March 2011

World Aquaculture 61
Microbial levan, an ideal prebiotic
and immunonutrient in aquaculture
S. K. Gupta, Pronob Das1*, S. K. Singh, M. S. Akhtar, D. K. Meena and S. C. Mandal
Aquaculture is one of the fastest growing agriculture
sectors in the world, providing food and nutritional secu-
rity to millions of people. However, disease outbreaks are a
constraint to aquaculture production, thereby affecting the
socio-economic status of people in many countries. For in-
stance, disease is now considered to be the limiting factor in
the shrimp farming industry. Disease control in the aquacul-
ture industry has been achieved using variou methods, in-
cluding traditional means, synthetic chemicals and antibiot-
ics. However, the use of such expensive chemotherapeutants
for controlling diseases has been widely criticized for their
negative effects, including accumulation of residues, devel-
opment of drug resistance, immunosuppression and reduced
consumer preference for products treated with antibiotics.
Traditional methods are ineffective against controlling new
diseases in large aquaculture systems. Therefore, alternative
methods need to be developed to maintain a healthy micro-
bial environment in aquaculture systems, thereby maintain-
ing the health of the cultured organisms. The use of prebiot-
ics, probiotics and immunonutrients is growing as means of
producing healthy organisms (Panigrahi and Azad 2007).
Prebiotics are non-digestible food ingredients that benefi-
cially affect the host by stimulating growth activity and/or
activity of a limited number of beneficial bacteria in the gas-
trointestinal tract (Gibson and Roberfroid 1995). According
to Panigrahi and Azad (2007), prebiotics are basically food
for probiotics. In the case of terrestrial animals, the use of
prebiotics has been progressing. Health-promoting bacteria
commonly augmented by prebiotics include those of the ge-
nus Lactobacillus and Bifidobacterium in terrestrial animals
(Manning and Gibson 2004). The use of prebiotics in aqua-
culture is a fairly recent development. Levan was reported
by Lippmann as early as 1881 and the name “Levulan” was
proposed for the compound. Microbial levan is used as a
prebiotic and immunonutrient in aquaculture.
The Chemical Structure of Microbial Levan
Levans are fructans, natural polymers of fructose. The
two main types of fructans are the levans with mostly ß
(2→6) linkages and the inulins with ß (2→1) linkages (Han
1990). Branched fructans with both types of linkages also
exist. Levan is the common name for a fructan in which
most fructose has ß (2→6) linkages. A more descriptive
name would be (2→6)-ß-D fructans. Lavans are homopoly-
saccharides composed of monomers of D-fructose attached
by ß (2→6) linkages that carry a D-glucosyl residue at the
end of the chain. They constitute a series of homologus oli-
gosaccharides and polysaccharides, which can be considered
derivatives of sucrose.
Although the structure of levan is represented by a
straight chain of ß (2→6) linkages, many bacterial levans
are branched through ß (2→1) bonds. The branch chains
are usually short and sometimes consist of one fructose resi-
due. In general, levans produced by different organisms have
similar structures. The difference may be a varying degree
of polymerization and branching of the repeating unit. Le-
vans are one of the few natural polymers in which the car-
bohydrate exists in the furanose form. This feature plays an
important role in the final confirmation of the molecules in
the solution.
Properties of Levan
Levan is a white crystalline powder having properties like
a strong adhesive. The particles are spherical shape, densely
packed, 75-200nm in diameter and do not swell in water,
which differs from other polysaccharides. The composition
and properties of levan depend upon the environment in
which the microorganisms are grown. The general properties
of levans resemble those of dextrans. Levans are levorota-
tory, amorphous or microcrystalline, of varying solubility in
cold water. They are very soluble in hot water and insoluble
in absolute ethyl alcohol. Levans are generally more soluble
than inulin, which is almost insoluble (<0.5 percent) in wa-
ter at room temperature. The high solubility of levan may
be a characteristic of the ß (2 →6) linkage compared to the
ß (2→1) linkage. Branching may be only a support factor.
Levans are non-reducing, not hydrolyzed by yeast invertase
and amylase action, but very susceptible to hydrolysis by
acid. They are not colored by iodine, but hydrogen chloride
imparts a purple color that distinguishes them from other
polysaccharides not containing fructose. The molecular
weight and viscosity in aqueous solution increases sharply
when various salts are present, but it decreases with a small
increase in temperature (Kang and Cottrell 1979).
Certain biological properties of levan, such as tumor in-
hibition and stimulation and increase in cell permeability for
a cytotoxic agent (Leibovici and Stark 1984) have attracted
attention. These effects are partly caused by suppression of
a normal inflammatory response. Only levan with Molecular
weight 107 promotes infection; the effect is lost when poly-
mers degrade. Levan, given intravenously to mice, greatly
increases the virulence of intraperitonially injected bacteria.
This is partly caused by permeability and the prevention of
the escape of blood constituents into the peritoneal cavity.