Effects of dietary probiotics on juvenile Nile tilapia

Tilapia is a significant species of farmed freshwater aquaculture. Disease caused by Streptococcus agalactiae has become a major challenge, however, resulting in massive losses for tilapia farmers worldwide.

Probiotics, particularly antagonistic probiotics, can reduce pathogenic bacteria by competitive exclusion, provide nutrients and enzymes to promote host growth, enhance the immune response by immune stimulation, and do not cause secondary pollution problems.

Obtaining S. agalactiae antagonising probiotics suitable for tilapia culture is therefore of great practical significance for improving the resistance of tilapia and reducing the use of antibiotics, according to a study published in Science Direct.

The study

In this study, a total of 720 apparently healthy juvenile Nile tilapia (0.20 ± 0.05 g) were randomly divided into 4 equal groups with 3 replicates for each group. Fish were fed a basal diet (control check group, CK group) supplemented with B. subtilis (1 × 10⁸ CFU/g feed, BS group), B. cereus NY5 (1 × 10⁸ CFU/g feed, BC group), and B. subtilis + B. cereus NY5 (0.5 × 10⁸ CFU/g feed of each probiotic, BS+BC group) for 6 weeks, and the probiotic supplementation groups were then fed the basal diet for 1 week to investigate the gut microbial community.

The results

The results of this study showed that:

• BS+BC and BC treatments significantly increased weight gain, feed conversion ratio and S. agalactiae resistance in Nile tilapia (P < 0.05).
• Gut microvilli length and density and c-type lysozyme gene expression were significantly increased by probiotic supplementation (P < 0.05).

The results of high-throughput sequencing showed that:

B. cereus NY5 and B. subtilis + B. cereus NY5-supplemented feed resulted in a significant improvement in tilapia autochthonous gut bacterial communities and had a stimulation effect on a variety of potential probiotics after 6 weeks of feeding.

After cessation of probiotic administration for 1 week, the gut bacteria of the fish in the BS+BC and BC groups had minor changes and maintained a stable state. Consequently, it was inferred that, as a feed supplement, B. cereus NY5 and the mixture of B. subtilis and B. cereus NY5 at 1 × 10⁸ CFU/g feed were able to promote growth and disease resistance, which may be associated with the supplement’s effects on gut immune status, intestinal morphology, and intestinal microbial community composition.