Abstract
Today, a high global demand for food security and food safety has caused rapid development in the aquaculture industry. Research on the efficacy and potency of Bacillus in the form of probiotics on growth performance and immunity is inadequate. The diverse physiological characteristics of Bacillus bacteria can provide a suitable condition in fish gastrointestinal tract resulting in better digestion and absorption of feed nutrients by fish, with an ultimate improvement in the animal growth performance. The detailed mode of action by bacilli bacteria as the major producers of proteinaceous substances, however, required more investigations. The probiotic bacilli can change the ecology of the gut microbiota of recipient fish via bacterial competition that can result in the prevention of pathogen attachment to the intestinal mucosa, ultimately improving of growth and health status of the target fish. In addition, Bacillus probiotics can modulate innate immune responses including lysozyme and phagocytic activity, anti-protease, respiratory burst, and antioxidant enzymes such as peroxidase, superoxide dismutase, and myeloperoxidase through an enhancement in some fish immunocompetent cell populations, for example, leukocytes, lymphocytes, monocytes, goblet cells, and erythrocytes. In addition, Bacillus probiotics can elicit changes in animal cell physiology, for example, increasing neutrophil adherence capacity, neutrophil migration, and plasma bactericidal activity that ultimately can result in the enhancement of immune effector functions such as enhancement in complement activity, immunoglobulin production, and cell cytotoxicity. The studies of immune-stimulatory effects by Bacillus spp. take place in the gut-associated lymphoid tissue of fish. However, detailed mechanisms of such studies warranted future research works. This chapter addressed the efficacy and potency of bacilli probiotics on growth immunity and disease resistance of various fish species and discussed the present gaps.