Synbiotic microcapsules of Bacillus subtilis and oat β-glucan on the growth, microbiota, and immunity of Nile tilapia

Survival of probiotics in processed feed can be affected not only by feed processing techniques but also by factors such as storage and gastrointestinal transit of ingested feed. This study investigates the effect of free and spray-dried Bacillus subtilis (BS) in oat β-glucan microcapsules on growth performance, hematology, intestinal microbiota, histology, and immunology of Nile tilapia, Oreochromis niloticus . A total of 400 tilapia were randomly stocked into 25 aquaria in a randomized experimental design and fed one of the following diets: (C) control diet (without synbiotics); (0.1%S) 1g kg −1 diet of unloaded microcapsules and 2.7 × 10 9 of free BS CFU kg −1 diet; (0.2%S) 2 g kg −1 diet of unloaded microcapsules and 5.4 × 10 9 of free BS CFU kg −1 diet; (0.1%SM) 1g of microcapsules loaded with 2.7 × 10 9 BS CFU kg −1 diet; and (0.2%SM) 2g of microcapsules loaded with 5.4 × 10 9 BS CFU kg −1 diet. After a 60-day feeding trial, fish fed 0.2%SM showed the highest growth performance and best feed utilization compared to fish fed free probiotics and control diet. Intestinal villi were longer, and the submucosa layer was thicker in fish fed both free and microencapsulated probiotics than the control group. Fusobacteriota, Firmicutes, and Bacteroidota were the dominant phyla across all samples accounting for more than 90% of the gut microbiota. No differences were registered in hematological parameters. Phagocytic activity was enhanced in fish fed both 0.2%S and 0.2%SM diets. Microencapsulation has the potential to protect B. subtilis and may constitute a valuable approach for enhancing the viability of probiotics as additives for fish feeds.