Studies on Bacillus subtilis and Lactobacillus acidophilus, as potential probiotics, on the immune response and resistance of Tilapia nilotica (Oreochromis niloticus) to challenge infections

Abstract

The probiotic activity of two bacteria (<i>Bacillus subtilis</i> and <i>Lactobacillus acidophilus</i>) was evaluated by its effect on the immune response of Nile tilapia (<i>Oreochromis niloticus</i>), beside its protective effect against challenge infections. Furthermore, their in-vitro inhibitory activity was evaluated. The in-vitro antimicrobial assay showed that <i>Bacillus subtilis</i> and <i>Lactobacillus acidophilus</i> inhibited the growth of <i>A. hydrophila</i>. The <i>B. subtilis</i> inhibited the development of <i>P. fluorescens</i> while <i>L. acidophilus</i> inhibited the growth of <i>Strept. iniae</i>. The <i>B. subtilis</i> and <i>L. acidophilus</i> proved harmless when injected in the <i>O. niloticus</i>. The feed, containing a mixture of <i>B. subtilis</i> and <i>L. acidophilus</i> or <i>B. subtilis</i> alone, showed significantly greater numbers of viable cells than feed containing <i>L. acidophilus</i> only after 1, 2, 3 and 4 weeks of storage at 4°C and 25°C. The survival rate and the body-weight gain were significantly increased in the fish given <i>B. subtilis</i> and <i>L. acidophilus</i> for one and two months after application. The hematocrit values showed a significant increase in the group that received the mixture of <i>B. subtilis</i> and <i>L. acidophilus</i> compared with the control group. The nitroblue tetrazolium (NBT) assay, neutrophil adherence and lysozyme activity, showed a significant increase in all the probiotic-treated groups after 1 and 2 months of feeding, when compared with the untreated control group. The serum bactericidal activity was high in the group that was given a mixture of the two bacteria. The relative level of protection (RLP) was significantly higher against <i>A. hydrophila</i>, in the bacterial mixture treated group and against <i>P. fluorescens</i> in the <i>L. acidophilus</i> treated group, after one month of the feeding trial. A significantly higher RLP, against <i>A. hydrophila</i> or ,<i>P. fluorescens</i>, was noticed after 2 months of the feeding trial in the group given a mixture of the two bacteria, and against <i>Strept. iniae</i> in the group fed a diet containing <i>L. acidophilus</i>.