Dietary effect of heat-inactivated Bacillus subtilis on the growth performance, blood biochemistry, immunity, and antioxidative response of striped catfish (Pangasianodon hypophthalmus)

Paraprobiotics confer potential replacements for live probiotics due to their beneficial effects on the performance of aquatic animals. Thus, this study tested the effects of heat-inactivated Bacillus subtilis (HIB) on the growth performance, digestion capacity, blood biochemistry, immunity, and antioxidative response of striped catfish. The HIB cells were mixed well with the basal diet at 0, 0.5, 1, and 2 g/kg, then fed to striped catfish for 60 days. The growth performance (final weight, weight gain, specific growth rate, and protein efficiency ratio) was enhanced, while the feed conversion ratio was reduced in striped catfish fed HIB at 1 and 2 g/kg (P < 0.05). Furthermore, the fish fed 1 and 2 g/kg HIB had higher protease activity than those fed the HIB-free diet (P < 0.05). Dietary HIB enhanced the intestinal histological appearance and increased the branching and length of the intestinal villi as well as immune cell penetration beneath the intestinal mucosa, particularly at 0.5 and 1 g/kg. Striped catfish fed dietary HIB at 1 g/kg had higher total protein and albumin than those fed an HIB-free diet, without a significant difference from those fed 0.5 and 2 g/kg (P > 0.05). The lysozyme and phagocytosis revealed increased activities in striped catfish provided dietary HIB at 1 and 2 g/kg (P < 0.05). The pooled results of the antioxidative status of striped catfish fed dietary HIB showed enhanced superoxide dismutase, catalase, and glutathione peroxidase and reduced malondialdehyde (MDA) levels. The regression analysis illustrated that HIB can be supplemented in the diets of striped catfish at 1.42–1.66 g/kg.