immunostimulants from marine yeasts are better than those from land yeasts at fighting marine diseases

Sukumaran V. Lowman D.
Sajeevan T. Philip R. (2010) Marine yeast glucans confer better protection
than that of baker’s yeast in Penaeus monodon against white spot syndrome virus
infection. Aquaculture Research, 41, 1799-1805.

Immunostimulants to combat virus
infection have been reported successful. This study finds that glucans from
marine yeast are effective at reducing mortalities of the giant tiger prawn Penaeus
monodon post larvae to white spot syndrome infection.

Glucans are the major structural polysaccharides in
the cell wall of most fungi and are also found in yeasts. In this study glucan
isolates from three marine yeasts (Debaryomyces hansenii (S8), Debaryomyces
hansenii (S169) and Candida tropicalis (S186)) and one from Baker’s yeast
(Saccharomyces cerevisiae (S36)) were examined for potential application as
immunostimulants. The shrimp were fed each glucan for 22 days before challenge
with white spot syndrome. The diets were maintained and survival was recorded
for the next seven days.

The glucans stimulate macrophage activity that
enhances non-specific defense mechanisms against the attack of infectious
agents. How this works was not explained but someone must know. The paper cited
a number of studies that had showed enhancement of the immunity to a range of
diseases in shrimp. The study found that glucan from C. tropicalis (S186)
supported maximum survival (69%) followed by the other two marine yeast (S169
and S8) glucans of 27% and 23% respectively while glucan from Baker’s yeast, S.
cerevisiae S36 exhibited poor survival (4%) in P. monodon post
challenge.

The significant
findings of this paper were that the differences in survival were due to
differences in the molecular weight and branching of the glucan molecules from
each source. The highest molecular weight and the lowest level of branching
produced the best results and visa versa. This can therefore be applied to
future studies when selecting glucan as an immunostimulant. The authors warn
that composition in terms of properties such as side-chain composition and
molecular weight of a yeast glucan isolate may vary based on the nature of the
extraction procedure and the yeast source. Immunostimulants are already being
used commercially but how effective they can be to abolish infections by
opportunistic pathogens remains to be seen as this study was over a short
period of time and the actual percentage survival was only impressive for one
glucan source. The authors also acknowledge that other similar studies have
contrasting findings and that considerable uncertainty exists regarding the
structure - activity relationship of glucan as immunostimulants.