Evaluation of the prebiotic GroBiotic (R)-A and brewers yeast as dietary supplements for sub-adult hybrid striped bass (Morone chrysops x M-saxatilis) challenged in situ with Mycobacterium marinum
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Dietary supplements such as immunostimulants and prebiotics hold promise as a potential replacement of antibiotics in maintaining fish health. A 21-week feeding trial was conducted to evaluate the commercial prebiotic GroBiotic-A, a mixture of partially autolyzed brewers yeast, dairy ingredient components and dried fermentation products, in the diet of hybrid striped bass exposed to chronic mycobacterial infection caused by Mycobacterium marinum, as compared to partially autolyzed brewers yeast (Brewtech). The basal diet was formulated to contain 40% protein, 10% lipid and an estimated digestible energy level of 3.5 kcal/g. Supplements of 1 or 2% brewers yeast and 2% GroBiotic-A were singularly added to the basal diet and each diet was manufactured by extrusion processing with a twin-screw extruder. Each diet was fed to three replicate groups of small (initially averaging 64.5 g/fish) and one group of large (initially averaging 118 g/fish) hybrid striped bass in 1187-l circular tanks operated as a recirculating system. Fish were fed twice daily to apparent satiation and growth performance monitored for 16 weeks. An in situ infection of M. marinum became well established at week 16 such that fish were fed once daily and mortality was monitored for a total of 21 weeks. Enhanced growth performance was generally observed in fish fed diets supplemented with GroBiotic-A or brewers yeast compared to fish fed the basal diet throughout the feeding trial with significantly (P < 0.05) enhanced weight gain observed after 12 weeks of feeding. At the end of the feeding trial, fish fed 2% brewers yeast had significantly higher feed efficiency than fish fed the other diets. The in situ mycobacterial challenge employed in this experiment resulted in overall cumulative mortality of approximately 25%. Fish fed 2% GroBiotic-A had a significantly (P < 0.05) enhanced survival (80%) compared to the other treatments (72-73%) at the end of 21 weeks. It is concluded that dietary supplementation of 2% GroBiotic-A showed moderate but significant (P < 0.05) protection against mycobacterial infection. Dietary supplementation of partially autolyzed brewers yeast also may enhance growth performance under chronic infection of mycobacteria. 2005 Elsevier B.V. All rights reserved.
