Abstract The manipulation of intestinal bacterial flora through dietary supplementation of beneficial
microbes is a new approach not only from the nutrition point of view but also to overcome the
adverse effects of chemotherapeutants and lack of effective vaccines. The current study was performed
to assess the role of Saccharomyces cerevisiae as a whole yeast cell (Probiotic), its extract
(mannan-oligosaccharide - Prebiotic) and Pre-Probiotic mixture (Synbiotic) as growth promoters
and immunostimulants in cultured Oreochromis niloticus (O. niloticus). One hundred fifty-six O.
niloticus were divided into four groups fed on Probiotic, Prebiotic, synbiotic and basal diet for
two months. Treated fish groups showed significant improvement in growth performance and activation
of non-specific cellular/humoral immunological measures together with a relative enhancement
of resistance against challenged bacteria. Synbiotic feed additive has proved significant
enhancement of fish innate resistance against selected fish pathogens as well as positively increased
the growth performance of challenged fish.