Differences in amino acid catabolism by gut microbes with/without prebiotics inclusion in GDDY-based diet affect feed utilization in rainbow trout

There is the need to enhance feed efficiency and growth of rainbow trout to reduce production costs of cultured fish. This study conducted a 3 × 4 factorial experiment with three graded levels of grain distiller dried yeast (GDDY) protein (0%, 50%, 75%) as replacement for fishmeal and four different prebiotics inclusion levels (0% (control), 0.4%, 1% mannooligosaccharides (MOS), and 1% GroBiotic A). The feeding trial was conducted for 12 weeks during which fish were fed daily to apparent satiation. Growth of rainbow trout was not affected by replacement of fishmeal with GDDY, but feed conversion ratio (P < 0.0001) was greater at the highest level of GDDY inclusion. Increasing GDDY inclusion significantly increased feed intake (P < 0.00015), which resulted in poor feed utilization. Acetic (P = 0.1994), propionic (P = 0.8037), butyric (P = 0.6268), valeric (P = 0.5877), and isovaleric (P = 0.5919) acids profiles did not differ by diet nor with inclusion of MOS or GroBiotic A. Whole shotgun metagenomic analyses of the gastrointestinal tract (GIT) microbiota revealed enrichment in the fungal phyla Ascomycota and Basidiomycota and the bacterial phylum Actinobacteria in the GDDY-fed fish compared to those fed the control fishmeal-based diet, which may be reflective of the species endogenous in GDDY. Microbial genes involved in branched-chain amino acid metabolism (glutamate, glutamine, aspartate) (P = 0.028) and glutamate dehydrogenase clusters (P = 0.0192), were also elevated in the fish fed the 75% GDDY-based diet. The results from this study indicate the potential for microbially-mediated catabolism of the non-essential amino acids, and suggest this activity may significantly influence efficient utilization of dietary nitrogen in the yeast-based protein diet.