Effects of Bacillus subtilis Strains on Growth, Immune Parameters, and Streptococcus iniae Susceptibility in Nile Tilapia, Oreochromis niloticus

This study was conducted to evaluate the effects of probiotic-amended diets fed to juvenile Nile tilapia, Oreochromis niloticus, on growth and susceptibility to Streptococcus iniae infection. Fish (average weight 16.5 ± 0.2 g) were fed five diets formulated with Bacillus subtilis strains SB3086, SB3295, SB3615, or AP193 either individually or in combination of strains SB3086 and SB3615 at a targeted concentration of approximately 4 × 10⁷ colony-forming units (CFU)/g of feed or with a basal control diet with no additives for 21 d. After the 21-d growth trial, no significant difference in growth performance was observed with any probiotic-amended diet. Results from serum bactericidal activity showed a significant difference between treatments and the control (P = 0.0002), except for the SB3295-amended diet (P = 0.9020). Lysozyme activity was also significantly different in fish fed probiotic diets from those fed control diet (P = 0.0001). After 21 d of feeding, fish were challenged with S. iniae by intraperitoneal injection at a dosage of 8 × 10⁶ CFU per fish. Results from the challenge also showed a significant difference between treatments and control (P = 0.0001). Overall, fish fed with strain SB3615 showed the lowest percent mortality (44.0 ± 7.2%) and those fed the control diet showed the highest mortality (77.3 ± 7.0%). The combined feeding with strains SB3086 and SB3615 did not result in any significant difference in reducing mortality because of S. iniae infection in juvenile Nile tilapia when compared with the individual probiotic treatments.