Species of Bacillus are spore-forming bacteria that are resistant to aggressive physical and chemical conditions, with various species showing unusual physiological features enabling them to survive in various environmental conditions including fresh waters, marine sediments, desert sands, hot springs, Arctic soils, and the gastrointestinal tract of finfish and shellfish. They are able to rapidly replicate and tolerate a multitude of environmental conditions, giving a wide range of beneficial effects in the aquaculture sector. Application of Bacillus spp. as probiotics in feed or for bioremediation of aquaculture rearing water has great potential for sustainable aquaculture. Species of Bacillus may play a desirable role in removing waste products from aquaculture environments, maintaining optimum water quality, and reducing stress, which can lead to an improved immuno-physiological balance, better growth and enhanced survival in target aquatic animals. Application of probiotic Bacillus spp. can enhance growth and immune function of aquatic organisms. Probiotic Bacillus can also assist in maintaining a higher density of beneficial bacteria and a lower load of pathogenic agents in aquaculture ponds. Much is still unknown, however, about how the probiotic efficacy of specific Bacillus species is affected by different aquatic animal species, age and growth condition, water quality, and diet. Also, the details of mode of actions of Bacillus spp. on the immune-physiological functions of aquatic organisms as well as their functions as bioremediators of water quality need further studies. This review addresses the presence of Bacillus spp. in the gastrointestinal tract of finfish and shellfish, their ability to produce enzymes and antibacterial compounds, and their efficacy and potency as probiotics in aquaculture.

The present paper presents an overview of the effects of gut microbiota and probiotics on lipid-, carbohydrate-, protein- and amino acid metabolism in fish and shrimp. In probiotic fish studies, the zebrafish (Danio rerio) model is the most frequently used, and probiotic administration reveals the effect on glucose homeostasis, anti-lipidemic effects and increasing short-chain fatty acids, and increased expressions of genes related to carbohydrate metabolism and innate immunity, along with down-regulation of oxidative stress-related genes. Further, improved length of the intestinal villi and expression of nutrient transporters in fish owing to probiotics exposure have been documented. The present review will present an appraisal of the effect of intestinal microbiota and probiotic administration on the metabolism of nutrients and metabolites related to stress and immunity in diverse fish- and shrimp species. Furthermore, to give the reader satisfactory information on the topics discussed, some information from endothermic animals is also presented.