Prebiotics Have Limited Effects on Nutrients Digestibility of a Soybean-Meal-Based Diet by Goldfish Carassius auratus
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Prebiotic compounds comprise a group of dietary supplements defined as nonviable food ingredients that are selectively metabolized to favor beneficial intestinal bacteria. Such bacteria may confer various desirable effects including enhanced disease resistance and nutrient availability to the host. This study examined the effects of four prebiotics, GroBiotic?-A (a mixture of partially autolyzed brewers yeast, dairy ingredient components and dried fermentation products), mannanoligosaccharide (MOS), galactooligosaccharide (GOS), and the fructooligosaccharide (FOS) inulin on digestibility of soybean-meal-based diets by goldfish. A basal diet was formulated so that 50% of the protein was provided by soybean meal and the other 50% was from menhaden fishmeal. Each prebiotic was supplemented to the basal diet at 1% by weight. A diet containing all of its protein from menhaden fish meal also was prepared as a control diet. Chromic oxide was added to the diets at 1% as an inert marker. Each diet was fed to adult goldfish in duplicate 110-L iv aquaria for a total of 8 weeks. The dried fecal material from each aquarium was pooled over time and analyzed for protein, lipid, organic matter and chromium in order to compute coefficients of apparent digestibility. Genomic DNA of gut microbiota also was isolated from the fecal samples of goldfish fed the various diets and subjected to polymerase chain reaction (PCR) using bacteria-specific PCR primers to conserved regions flanking the variable V3 region of 16S rDNA. Then, denaturing gradient gel electrophoresis (DGGE) of the resulting amplicons was conducted as a means of assessing diversity of microbiota in the gastrointestinal (GI) tract. Results of the present study revealed that none of the prebiotics affected apparent digestibility coefficients of the soybean-meal-based diet compared to the basal diet, although the diet supplemented with MOS consistently yielded the lowest values. In addition, goldfish digested the soybean-meal-based diets as well as the control diet. DGGE analysis revealed no differences in microbiota of goldfish fed the various prebiotics. These results are in contrast to those obtained with carnivorous fish species such as the red drum (Sciaenops ocellatus) in which the prebiotics increased digestibility coefficients of soybean-meal-based diets and altered GI tract microbiota.