Dietary brewers yeast and the prebiotic Grobiotic™ AE influence growth performance, immune responses and resistance of hybrid striped bass (Morone chrysops x M-saxatilis) to Streptococcus iniae infection

Use of prebiotics, nondigestible dietary ingredients that beneficially affect the host by selectively stimulating the growth of and/or activating the metabolism of health-promoting bacteria in the intestinal tract, is a novel concept in aquaculture. Two separate feeding trials were conducted to evaluate graded levels of a commercial prebiotic Grobiotic(TM) AE, a mixture of partially autolyzed brewers yeast, dairy ingredient components and dried fermentation products, in the diet of hybrid striped bass, as compared to partially autolyzed brewers yeast (Brewtech(R)). The basal diet in both trials was formulated to contain 40% protein, 10% lipid and an estimated digestible energy level of 3.5 kcal/g. Two levels (1% and 2% of diet) of Grobiotic(TM) AE and brewers yeast were added to the basal diet with menhaden fish meal and menhaden oil adjusted to provide isonitrogenous and isolipidic diets. Each diet was fed to five (trial 1) or three (trial 2) replicate groups of juvenile hybrid striped bass in 110-1 aquaria twice daily at rates approximating apparent satiation for 7 weeks (trial 1) or 4 weeks (trial 2). Enhanced growth performance was generally observed in fish fed the diets supplemented with Grobiotic(TM) AE or brewers yeast compared to the basal diet after 7 weeks of feeding in trial 1. Significantly higher (P< 0.05) feed efficiency was observed in fish fed diets supplemented with 1% and 2% Grobiotic(TM) AE. After 4 weeks of feeding in trial 2, growth and feed efficiency were not significantly affected by the various dietary treatments, although some immunological responses were altered. Neutrophil oxidative radical anion production and intracellular superoxide anion production of head kidney macrophages tended to be higher in fish fed diets supplemented with brewers yeast or Grobiotic(TM) AE, while extracellular superoxide anion production of head kidney macrophages from fish fed diets with 1% and 2% brewers yeast and 1% Grobiotic(TM) AE was significantly (P<0.01) higher than that of fish fed the basal diet. All groups of fish fed brewers yeast and Cirobiotic(TM) AE showed a significantly (P<0.01) enhanced survival (73.3-90%) after bath exposure to Streptococcus iniae compared to fish fed the basal diet (53.3%). Based on these data, it is concluded that Grobiotic(TM) AE and a partially autolyzed brewers yeast can serve as functional feedstuffs in the diets of hybrid striped bass by enhancing growth performance and immunological responses. Further research is needed into the mechanism(s) of action for prebiotics such as Grobiotic(TM) AE and their application in aquaculture. (C) 2004 Elsevier B.V. All rights reserved.