Improvement in growth performance, digestive enzymes activity, immune-related gene expression, and resistance against Edwardsiella tarda ETS1.1 in African catfish (Clarias sp.) with dietary supplementation of euryhaline probiotic and paraprobiotic Bacillus cereus BR2

Abstract The use of probiotics has become an alternative in the aquaculture industry and is considered a new tool for disease control. However, the maintenance of viability of probiotic bacterial cells during storage is quite challenging. This study aimed to investigate the potential effects of euryhaline probiotic and paraprobiotic B. cereus BR2 on African catfish, specifically on growth performance, gastrointestinal health, immune-related gene expression, and resistance to E. tarda ETS1.1. The test feed was enriched with a 1% (w/w) dose of probiotics and paraprobiotics at a cells density of 10⁸ and 10¹⁰ CFU mL⁻ ¹, each with replications. For diet supplementation experiment, African catfish fry with initial average body weight of 6.67 ± 0.34 g were randomly placed into six aquaria containing 60 L volume of freshwater and reared for 30 days. Afterwards, fish were challenged intraperitoneally on day 31− th with E. tarda ETS1.1 at cell density of 10⁷ CFU mL⁻ ¹, and reared for 14 days. The results showed that diets treated with probiotics and paraprobiotics 10⁸ and 10¹⁰ CFU mL⁻ ¹ significantly (p < 0.05) increased mean final weight, mean final biomass, specific growth rate (SGR), disease resistance (survival rate) and reduced feed conversion ratio (FCR) values. Digestive enzyme (amylase, protease, lipase) activities, total bacterial population and abundance of B. cereus BR2 in the intestine were measured higher in supplementation treatments than control fish. After the challenge test, decreased E. tarda ETS1.1 abundance in kidney and liver of fish were measured in both probiotic and paraprobiotic treatments. Furthermore, interleukin 1β (IL-1β) gene expression showed upregulation in fish after treatment. Similarly, major histocompatibility complex 2β (MHC-2β) expression was significantly upregulated after supplementation and post-challenge, with the highest level of MHC-2β mRNA expression found in DPI 7. The supplementation of probiotic and paraprobiotic B. cereus BR2 per oral effectively improved growth performance, digestive enzymes activity, immune-related gene expression, resistance of African catfish to E. tarda ETS1.1.