29Bacillus are Gram-positive spore-forming bacteria that are resistant to aggressive physical and 30 chemical conditions with various species existing unusual physiological features enabling them 31 to survive in various environmental conditions including fresh waters, marine sediments, desert 32 83 beneficial bacteria such as genus Bacillus in the GI tract of aquatic organisms merits 84 investigations, as autochthonous bacteria, adherent to mucosa, rapidly colonise the digestive 85 tract at early developmental larval stages (Ringø et al. 1996; Hansen & Olafsen 1999; Vadstein 86 et al. 2018).87 4 SOLTANI ET AL.During the last 20 years, numerous studies have been published on Bacillus in the intestine of 88 finfish and shellfish, their potential as probiotics, pathogenicity and their effect on the immune 89 system (e.g. Abriouel et al. 2011; Cha et al. 2013; Kim et al. 2014; Mukherjee & Ghosh 2016; 90 Wu et al., 2016; Sreenivasulu et al., 2016; Dash et al., 2018; Bachruddin et al. 2018). 91Therefore, the current review aimed to present an overview of published data on Bacillus in the 92 GI tract of finfish and shellfish, antagonistic ability, health benefits as probiotics, pathogenicity 93 and their effect on immunostimulation. 94 Bacillus in the gastrointestinal (GI) tract of finfish and shellfish 95Optimal GI functionality is of importance for sustainable animal production, and three 96 components namely diet, mucosa and commensal microbiota are essential for gut health. 97 Evaluation of the gut microbiota of finfish dates back to late 1920`s and early 1930`s (Reed & 98 Spence 1929; Gibbons 1933) and sine then numerous studies have been published. The GI tract 99 microbiota in fish is divided into; the GI lumen microbiota (the allochthonous), and those that 100 adhere to the mucosal surface (the autochthonous microbiota), however, several factors can 101 modulate the gut microbiota (Ringø et al. 2016). The intestinal microbiota contributes to several 102 essential factors such as pathogen displacement, nutrient-and receptor competition, production 103 of antimicrobial factors, structural factors (induction of IgA, tightening of tight junction and 104 immune system development) and several metabolic functions. Of the commensal gut 105 microbiota of aquatic animals, phylum Firmicutes which include genus Bacillus is essential, 106 and the contribution of Sugita and colleges (Japanese scientists) and Indian scientists (Ghosh 107 and colleges, and Ray and colleges) can not be underestimated.108

Shrimp aquaculture is the fastest growing animal food producing sectors in the world due to its high demand in developed countries; however, they are affected by diseases mainly caused by opportunistic pathogens results in huge economic losses. Today preventive and management measures are central concern to overcome such outbreak of diseases. The adverse effect of chemotherapeutics in shrimp farming has drawn attention for the development of non-antibiotic and environment friendly agents, which is one of the key factors for health management in shrimp aquaculture. In recent years, application of probiotics against virus and bacteria in shrimp farming had been a novel and safe approach since they promote the innate immune response. Probiotics for bacterial diseases like vibriosis is well reported, but for viral diseases, the authentic strains still need research. Apart from the discovery of new or better formulations, improvement in the probiotic benefits may be helpful. Thus, better and economic production methods, administration ways or combination with other preventive/therapeutic measures are welcomed. Further studies are still necessary to increase the knowledge about use of probiotics to control bacterial infections in shrimp but much more efforts are needed in the case of viral diseases. In this review, potential application of probiotics in shrimp aquaculture: importance, mechanisms of action and methods of administration are being focused. The article also explains the limitations and prospects of probiotics in sustainable and ecofriendly shrimp culture to augment the total shrimp aquaculture production.

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Shrimp, as a high-protein animal food commodity, are one of the fastest growing food producing sectors in the world. It has emerged as a highly traded seafood product, currently exceeding 8 MT of high value. However, disease outbreaks, which are considered as the primary cause of production loss in shrimp farming, have moved to the forefront in recent years and brought socio-economic and environmental unsustainability to the shrimp aquaculture industry. Acute hepatopancreatic necrosis disease (AHPND), caused by Vibrio spp., is a relatively new farmed penaeid shrimp bacterial disease. The shrimp production in AHPND affected regions has dropped to ~60%, and the disease has caused a global loss of USD 43 billion to the shrimp farming industry. The conventional approaches, such as antibiotics and disinfectants, often applied for the mitigation or cure of AHPND, have had limited success. Additionally, their usage has been associated with alteration of host gut microbiota and immunity and development of antibiotic resistance in bacterial pathogens. For example, the Mexico AHPND-causing V. parahaemolyticus strain (13-306D/4 and 13-511/A1) were reported to carry tetB gene coding for tetracycline resistance gene, and V. campbellii from China was found to carry multiple antibiotic resistance genes. As a consequence, there is an urgent need to thoroughly understand the virulence mechanism of AHPND-causing Vibrio spp. and develop novel management strategies to control AHPND in shrimp aquaculture, that will be crucially important to ensure food security in the future and offer economic stability to farmers. In this review, the most important findings of AHPND are highlighted, discussed and put in perspective, and some directions for future research are presented.