Immune stimulation and improved infection resistance in European sea bass (Dicentrarchus labrax) fed mannan oligosaccharides

The objective of this study was to determine the effect of two levels of inclusion of mannan oligosaccharides derived from the outer cell wall of a select strain of Saccharomyces cerevisiae (Bio-Mos, Alltech Inc, USA) on growth, feed utilization, immune status and disease resistance of European sea bass (Dicentrarchns labrax). Specimens of 35 g at initial density of 3 kg/m(3) were fed during 67 days at 0 parts per thousand, 27 parts per thousand and 47 parts per thousand dietary MOS level of inclusion in a commercial sea bass diet. Food conversion rate, specific growth rate, whole body biochemical composition, phagocyctic index of head kidney macrophages, NBT index, lysozyme and alternative complement pathway (ACP) activities as well as gut and liver histological structure were evaluated. Growth significantly increased at both MOS dietary inclusion levels. Histological features of the liver showed lower lipid vacuolization and regular-shaped morphology of hepatocytes around the sinusoidal spaces denoting a better utilization of dietary nutrients. No differences were found on gut histological evaluation. Statistical differences (P < 0.05) on the phagocytic index were denoted with the inclusion of 47 parts per thousand Bio-Mos group. A positive correlation was found between the levels of lysozyme and alternative complement pathway activities in blood and the level of inclusion of MOS in diets. After the feeding trial, a cohabitation challenge test and direct gut inoculation were also performed with the pathogen Vibrio alginolyticus in a ratio 3:1. Twenty-one days post-challenge the number of cohabitant fish infected in the control group reached 33% comparing with none on the 0.4 parts per thousand MOS group. Finally, new fish were infected with V. alginolyticus by gut canalisation. After 24 h post-infection no significant difference was denoted between groups and 48 h post-infection total infected fish in the control croup was twice that of the 2 parts per thousand and 4 parts per thousand MOS groups. (C) 2007 Elsevier Ltd. All rights reserved.