Abstract

The farming of Nile tilapia (Oreochromis niloticus) has faced the challenges of infections, especially by Streptococcus However, solving this problem by using antibiotics will cause other problems, such as drug residue that can directly affect consumers and an increase in antibiotic resistance to bacteria. Therefore, probiotics should be an alternative to control bacterial diseases. This study aimed to investigate the optimum level of mixed probiotic supplementation. The experiment was designed to include six treatments (the control diet and probiotic-supplemented diets at 0.5, 1.0, 2.0, 5.0 and 10.0%). The mixed probiotic was composed of 3.1 × 108 CFU of Lactobacillus sp. and 4.6 × 106 CFU of Saccharomyces cerevisiae. After feeding for 30 days, tilapias from each replication were injected with a vaccine containing Streptococcus agalactiae to study the immune gene expression. The results showed that this mixed probiotic can enhance the growth and immune system of tilapias, and the most effective level was at 0.5% with consideration for high ADG, low FCR, and high protective efficiency against S. agalactiae infection. Moreover, toll-like receptor 9 gene and immunoglobulin gene expressions after vaccination for 24 hours were significantly higher than the control. The relative percent survival was the highest at 78.13%. This mixed probiotic can be used for Streptococcosis control in tilapia farms because it can reduce the diet cost and induce both the growth and immune system of tilapias.