Probiotic mix of Bacillus spp. and benzoic organic acid as growth promoter against Streptococcus agalactiae in Nile tilapia

The aim of this study was to evaluate the effects of a probiotic mix composed of Bacillus subtilis, B. licheniformis and benzoic acid, on the zootechnical performance and hematoimmunological parameters of juvenile Nile tilapias; and to evaluate survival and immunological parameters after infection with Streptococcus agalactiae via gavage. A total of 320 fish were distributed into four experimental groups with four replicates (20 fish per experimental unit). These were fed as follows for 54 days: control (no supplementation); feed supplemented with 0.1% benzoic acid (BA0.1%); feed supplemented with a probiotic mix of Bacillus spp. (B); and feed supplemented with Bacillus spp. together with 0.1% benzoic acid (B + BA0.1%). Fish fed with BA0.1% showed better results for weight gain (48.33 +/- 5.23 g), final biomass (1005.99 +/- 85.10 g), specific growth rate (4.14 +/- 0.08) and survival after experimental challenge, compared with the control group. The number of thrombocytes was positively influenced in all treatments (B), compared with the control group. Combined supplementation of the probiotic mix of Bacillus spp. and 0.1% benzoic acid promoted a synergistic effect on the concentration of total leukocytes, compared with supplementation only with BA0.1%. Agglutination activity was higher post-infection (p < 0.05) in the group supplemented with Bacillus probiotic mix, compared with BA0.1% e B + BA0.1%. It is emphasized that 0.1% benzoic acid supplementation provided better zootechnical performance and survival for Nile tilapia in the challenge with S. agalactiae.