Effects of dietary Lactobacillus reuteri on antimicrobial activity, antioxidant capacity, non-specific immune response, and resistance to Streptococcus agalactiae infection of the Nile tilapia (Oreochromis niloticus)

The aim of the present study was to investigate the effects of dietary Lactobacillus reuteri on the antimicrobial activity, antioxidant capacity, immune response, and disease resistance against Streptococcus agalactiae of the Nile tilapia (Oreochromis niloticus). The results of in vitro antibacterial assay demonstrated that L. reuteri had antibacterial effects against the main pathogenic bacteria of tilapia, S. agalactiae and Aeromonas hydrophila. A feeding trial was next conducted, fingerling tilapia were fed with basal diet (control [CT]) supplemented with L. reuteri at concentrations of 109 (R9), 1010 (R10), and 1011 (R11) colony-forming units /kg for 56 days. Dietary L. reuteri significantly decreased blood glucose (p < 0.05) and significantly increased LZM activity of serum in R10 and R11 groups as compared to the CT group. In addition, the activity of glutathione peroxidase was decreased in R11 group, the level of malondialdehyde was decreased and the level of glutathione was increased in R10 and R11 group, and the level of total antioxidant capacity was increased in R9, R10 and R11 groups (p < 0.05). As compared to the CT group, the transcriptional level of TGF-beta was significantly increased in liver samples of the R10 and R11 groups (p < 0.05). In the intestinal mucosa, mRNA expression of TGF-beta was significantly upregulated, while that of IL-1 beta was significantly downregulated in the R10 and R11 groups (p < 0.05). Moreover, the survival rate was significantly higher in tilapia fed L. reuteri than the CT group after challenge with S. agalactiae. The relative percent survival of the R10 and R11 groups compared to CT group was 35.30%, and the RPS of group R9 compared to group CT was 23.53%. These results demonstrate that dietary L. reuteri enhanced antioxidant capacity, regulated immune-related genes and improved resistance against S. agalactiae in tilapia.