Selenium (Se) is an essential trace element for humans and animals, which has a wide range of biological effects, including improving antioxidant capacity, immune response, reproductive performance, and Se accumulation, regulating body metabolism, anti-tumor, anti-free radicals, anti-aging and anti-toxic elements [1, 2]. Among them, it is the most prominent biological function for Se to maintain the balance of the redox state of the body [3, 4]. Se can inhibit the lipid peroxidation of cell membranes, thereby eliminating harmful free radicals, decomposing peroxides and repairing molecular damage[5, 6]. Se deficiency can reduce the activity of glutathione peroxidase (GPx), damage the biofilm of various tissues and organs, and cause apoptosis, degeneration, or necrosis [7, 8]. In humans, Se deficiency can lead to Keshan disease, Kashin-Beck disease, cancer and cardiovascular disease[9, 10]. In animals, Se deficiency can cause muscle degeneration, pancreas hypoplasia and degeneration, liver parenchyma malnutrition and necrosis, long-term digestive disorders, severe diarrhea, and even death[8]. In the world, China is one of the regions with the most serious Se deficiency. There are 22 provinces in the low-Se regions, accounting for 72% of the total area, and the Se-deficient population exceeds 700 million [2, 4]. Therefore, Se supplementation in diets and foods is one of the commonly used measures, and the most commonly used Se preparation today is inorganic Se, sodium selenite (SS). Unlike SS, which has low absorption rate, high toxicity, peroxidation and potential pollution problems, most of the organic Se sources have the advantages of low toxicity, high absorption rate, high bioavailability, and low environmental pollution [4, 11]. Therefore, the development of organic Se sources has become one of the current research hotspots.
Probiotics, a class of active microorganisms, are beneficial to the host, and have the functions of improving the intestinal microecological environment, promoting the digestion and metabolism of nutrients, enhancing the immunity of the body, and improving the growth performance of animals[12, 13]. Among them, Bacillus subtilis (B. subtilis) can form spores, and has the characteristics of strong stress resistance, high temperature and high pressure resistance, acid and alkali resistance, and storage resistance [14, 15]. Previous studies have found that B. subtilis can secret some antibacterial active substances, such as gramicidin and antibacterial peptides, which have obvious inhibitory effects on pathogenic bacteria or opportunistic pathogens of endogenous infection [16, 17]. B. subtilis can also synthesize enzymes such as amylase, protease, lipase and cellulase, and plays a role in the digestive tract together with the digestive enzymes in animals to supplement the lack of endogenous enzyme secretion in animals [18]. Moreover, B. subtilis can synthesize functional molecules such as multivitamins, proteins, polypeptides, polysaccharides and oligosaccharides during the metabolic process to promote animal growth and health [19]. Nowadays, B. subtilis has become one of the most widely used probiotics in animal husbandry.
B. subtilis itself contains a rich biological enzyme transformation system, which can convert inorganic Se into highly active organic Se, and plays the dual role of organic Se and probiotics, which is an ideal Se supplement [17, 18]. Some previous studies reported that dietary Se-BS supplementation could modify ileal bacterial composition [19, 20], improve Hg-induced intestinal microbial changes, alleviate the abundance of Aeromonas and the inflammation of the fish, and protect common carp against Hg toxicity [21]. Otherwise, the application of Se-BS in animal production is not extensive enough, the majority of scientific researchers need to conduct in-depth investigation on its biological effects such as its effectiveness and availability. Therefore, this study was to evaluate the effects of Se-BS on growth performance, antioxidant capacity, immunity and gut health of broilers in order to provide some reference materials for the future application of Se-BS.