EFFECT OF AUTOCHTHONOUS AND COMMERCIAL PROBIOTIC BACTERIA ON GROWTH, BACTERIAL PERSISTENCE, IMMUNITY, AND DISEASE RESISTANCE IN NILE TILAPIA Oreochromis niloticus

Probiotics are gaining worldwide acceptance as means to improve growth and health of fish. However, the efficacy of the applied probiotic depends on the type of probiotic, dose, and duration of application. This study compared the effect of feeding an autochthonous Bacillus subtilis (Gene Bank NCBI 1701438) isolated from the gut of cultured tilapia (TPI), a commercial Lactobacillus acidophilus probiotic (COM) and a 1:1 mixture of both (MIX) on mean weight (MWT), specific growth rate (SGR) feed conversion (FCR), gross yield (GY), persistence, immunity, and disease resistance with the probiotic-free diet control (CON).

Healthy Nile tilapia (28.3 g) were stocked in 0.43-m³ tanks at density of 4.68 kg/m³. Commercial 3.0-mm tilapia pellets (33% CP) were coated with the respective probiotic suspension having a bacterial load of 10⁸ cfu g^-1. The fish were fed daily at 5% of the BW decreased to 3% for the rest of the experiment (105 days). Temperature was maintained at 29.0 ± 2.0 ^oC.

After 105 days, the MIX group had the best growth values (P<0.05) followed by the TPI and the COM groups. The autochthonous TPI bacterium had the highest count in the gut and persistence rate followed by the MIX bacteria. The COM bacterium had the lowest count and disappeared rapidly from the gut once withdrawn from the diet (Table 1).

The TPI significantly increased phagocytosis of the head kidney leucocytes and hemagglutination titers (HAT). The COM and MIX probiotics resulted in significantly higher superoxide dismutase (SOD). Probiotics showed no significant effect on lysozyme activity (LA) and the alternative complement activity (ACH₅₀) (Table 1). Fish fed with TPI and challenged by injecting the LD₅₀ dose (10^4.86 cfu) of the opportunistic pathogenic bacteria Proteus vulgaris had the lowest mortality rate and highest (P <0.05) relative protective survival (RPS) followed by the MIX treatment and lastly by the commercial probiotics. The highest mortality was recorded in the untreated controls. (Table 1).

Results obtained in this study indicate the higher potential of the autochthonous probiotic bacteria and the mixture of bacteria in enhancing growth, immunity, and disease resistance in tilapia than the commercial products.