Efficacy of Bacillus spp. isolated from Nile tilapia Oreochromis niloticus Linn. on its growth and immunity, and control of pathogenic bacteria

Probiotics can act as biological control agents against bacterial infection in aquatic animals, and also increase growth and stimulate immunity. In this study, two potential probiotics, Bacillus sp. KUAQ1 and Bacillus sp. KUAQ2, isolated from Nile tilapia Oreochromis niloticus Linn. intestine, were investigated for their biological functions. Species of isolated bacteria were identified by a conventional microbiological assay, biochemical assay and 16S ribosomal RNA polymerase chain reaction analysis. Both Bacillus isolates could survive at a pH ranging from 2 to 9 for 6 h and for 2 h in bile salts. The candidate probiotics were tested for their inhibition activity against the pathogenic bacteria Streptococcus agalactiae and Aeromonas hydrophila, and their specific protease activity. The probiotics had no significant effect (P > 0.05) on the average weight, average daily growth, specific growth rate or feed conversion ratio of tilapia fry after an 8-week feeding trial. Furthermore, supplementation with probiotics did not increase the survival rate of tilapia challenged with S. agalactiae. Several immune parameters including lysozyme, phagocytic activity and respiratory burst activity of juvenile fish treated with probiotics were significantly higher than those of the control (P < 0.05), but levels of the superoxide anion and alternative complement activity did not significantly differ (P > 0.05). Stress tolerance to brackish water at 25 p.p.t. NaCl did not improve significantly in fish treated with probiotics (P > 0.05). These two probiotic Bacillus offer benefits in terms of disease control and stimulation of the immune response of cultured tilapia.