Effects of Bacillus subtilis strains and the prebiotic Previda® on growth, immune parameters and susceptibility to Aeromonas hydrophila infection in Nile tilapia, Oreochromis niloticus

This study was conducted to evaluate the individual and combined effects of long-term feeding of diets containing two probiotic Bacillus subtilis group strains (Aqua NZ and AP193) and the prebiotic Previda((R)), a commercial hemicellulose extract, on growth performance, immune parameters and Aeromonas hydrophila susceptibility of juvenile Nile tilapia, Oreochromis niloticus. Nile tilapia of average weight 7.47 +/- 0.11g were fed diets formulated with the probiotics and/or the prebiotic, or a control diet for 8weeks and, subsequently, challenged with A.hydrophila by intragastric gavage at a dosage of 3.9x10(7) CFU per fish. Fish attained a mean weight of 59.5 +/- 0.99g at the end of the growth period. Under the conditions of the present trial, none of the diets significantly improved mean per cent weight gain (P=0.70), thermal growth coefficient (P=0.88) or feed conversion ratio (P=0.87) of Nile tilapia. Except for the diet containing the prebiotic Previda((R)) only (P=0.17), all other diets resulted in significantly higher fish survival compared to the control (P<0.05). The combined effect of the prebiotic and probiotic strains emerged as the most important diet with respect to mortality reduction. The mean lysozyme and respiratory burst activities did not show any significant differences between treatments and control (P=0.14 and 0.32, respectively). Thus, these probiotic strains used in this study have the potential to prevent disease due to A.hydrophila in O.niloticus.