Dietary Inclusion of Halobacterium salinarum Modulates Growth Performances and Immune Responses in Farmed Gilthead Seabream (Sparus aurata L.)

SIMPLE SUMMARY: It is known that, in intensive farming, fish are often exposed to multiple sources of stress that increase their susceptibility to pathogens and promote disease outbreaks. The latter are recognized as a major constraint on sustainable animal production, which can cause significant economic losses in the aquaculture sector. Many drugs have traditionally been used for the treatment and prevention of diseases in farmed fish, but they are not recommended, because the continued overuse of antibiotics can lead to the development of antibiotic-resistant bacteria, environmental pollution and the accumulation of toxic residues in fish. Over the past decade, scientific research has focused on the isolation, identification and use of bioactive compounds (rich in carotenoids and other natural antioxidants) obtained from microalgae, plants and micro-organisms, with the goal to develop alternative dietary supplements that improve the growth performance, health and immune system of farmed fish. The reinforcement of fish defense mechanisms through the prophylactic administration of immunostimulants, probiotics, prebiotics and other natural substances is one of the most promising methods to improve the profitability of aquaculture by improving the immune response of fish and decreasing the risk associated with the use of chemicals. ABSTRACT: The use of natural immunostimulants is considered the most promising alternative to promote fish health, productive performance and quality, increasing the aquaculture profitability, sustainability and social acceptance. The purpose of this study was to evaluate the effect of the integration of a potential probiotic strain, Halobacterium salinarum, belonging to the Archaea domain, in the formulated diets of farmed gilthead seabream (Sparus aurata L.) in terms of growth performances and immunity responses. The experiment was set up to test two different levels of inclusion of the bacteria in the diet: 0.05% (D1) and 0.1% (D2). The effects on fish growth performances; humoral (peroxidase, protease, antiprotease and IgM levels) and cellular immunity parameters (phagocytosis, respiratory burst and myeloperoxidase), along with bactericidal activity, were evaluated after 15 and 30 days of experimental feeding. The obtained results showed that the inclusion of H. salinarum at the highest concentration (D2 0.1%) improved growth performances, bactericidal activity against Vibrio anguillarum and some parameters related both to the humoral and cellular immune response, suggesting exploring other aspects of welfare in view of future supplementations of this probiotic strain in the diet of S. aurata.