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The aims of this study were to explore the effect of a probiotic consortium at different concentrations (low, medium and high) on bacterial community structure and composition in the hepatopancreas and digestive tract of Pacific white shrimp Litopenaeus vannamei reared under superintensive conditions, as well as to investigate its effect on gene expression of antioxidant enzymes. After the 69-day experimental period, shrimp treated with high probiotic concentration showed higher final growth, specific growth rate, weight gain and survival (p <.05) than those treated with low probiotic concentration or without probiotics; however, there were no significant differences compared to those treated with medium probiotic concentration. The analysis of the bacterial community structure revealed significant differences among treatments with and without probiotics; however, there were no differences between hepatopancreas and digestive tract samples, independently of the treatment received. Moreover, there were no differences in those samples from shrimp treated with probiotics at medium and high concentrations. The analysis of the bacterial community composition showed a higher proportion of Alphaproteobacteria (particularly members belonging to the family Rhodobacteraceae) in hepatopancreas and digestive tract samples from shrimp treated with probiotics at medium and high concentrations than in those treated with low probiotic concentration or without probiotics. In contrast, a higher proportion of Gammaproteobacteria (particularly members belonging to the families Alteromonadaceae , Moraxellaceae , Pseudomonadaceae and Vibrionaceae) was found in both sample types from shrimp treated without probiotics than in those treated with probiotics, independently of the concentration tested. At the end of the study, the levels of MnSOD (manganese superoxide dismutase) and CAT (catalase) transcripts were similar among hepatopancreas samples from shrimp treated with or without probiotics; however, the levels of GPx (glutathione peroxidase) transcripts drastically increased in those samples from shrimp treated without probiotics. These results could explain, in part, the mechanisms (i.e., competitive exclusion and immunomodulation) whereby probiotics confer beneficial effects to the host. • A probiotic consortium of Bacillus species and Enterococcus faecalis improved growth performance in Pacific white shrimp. • The probiotic consortium modified bacterial communities in the shrimp hepatopancreas and digestive tract. • Levels of glutathione peroxidase (GPx) transcripts only increased in shrimp treated without probiotic consortium. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the effect of rice bran fermented with a consortium of Bacillus and Lysinibacillus strains (RB + BLb) on the bacterial community structure and composition in the gastrointestinal tract of Pacific white shrimp (Penaeus vannamei), as well as its effect on growth performance and health status. Rice bran fermented with a commercial probiotic (RB + Com) was also evaluated for comparative purposes. Rice bran with or without probiotics, pre-fermented for 24 h, was directly added to the rearing water of juvenile shrimp for 4 weeks. At the end of the experimental period, shrimp treated with RB + BLb had a slightly better shrimp growth performance and survival than the control group; however, the differences were not statistically significant (p > 0.05) among treatments. Moreover, histological analyses showed that shrimp treated with RB + BLb improved the nutritional status compared to other treatments, by inducing an increase in lipid reserves of hepatopancreas epithelial cells. The analysis of the bacterial community composition revealed that the most prevalent families were Rhodobacteraceae , Vibrionaceae and Flavobacteriaceae. Although sequences affiliated to the Rhodobacteraceae family were more abundant in shrimp treated RB + Com, a lower proportion of sequences affiliated to the Vibrionaceae family was found in shrimp treated with unfermented rice bran (RB) or RB + Com, when compared with the control group or those treated with RB + BLb. The abundance of sequences affiliated to the Flavobacteriaceae family was significantly lower in shrimp treated with RB + BLb or RB + Com than the control group or those treated with RB. These results could explain, in part, the mechanisms of action whereby the use of rice bran fermented with a consortium of probiotic bacteria confer beneficial effects on shrimp culture. Unlabelled Image • Rice bran fermented with a consortium of Bacillus and Lysinibacillus strains showed great potential as safe probiotics. • Fermented rice bran modified the bacterial communities in the hepatopancreas of shrimp. • Fermented rice bran offered a slightly better growth performance and survival than the control group. [ABSTRACT FROM AUTHOR]