Relief of hypersaline stress in Nile tilapia Oreochromis niloticus by dietary supplementation of a host-derived Bacillus subtilis strain

Bacillus subtilis E221 was successfully isolated from the intestine of Nile tilapia (Oreochromis niloticus) fed a 0.8% inulin diet for eight weeks under salinity of 16 psu (practical salinity units) as a potential host-derived probiotic strain. The B. subtilis E221 was screened through in vitro assays based on the properties of non-haemolytic activity, extracellular enzyme activity, tolerance to gastrointestinal stress and bile salts, cell surface hydrophobicity, auto-aggregation and biofilm formation. Dietary B. subtilis E221 (1.0 × 10⁸ CFU·g⁻¹) supplementation significantly improved the weight gain, specific growth rate and feed conversion ratio of O. niloticus at 16 psu after eight-week feeding trial. The content of serum 50% haemolytic complement in fish fed the B. subtilis E221 was significantly higher than that of the control group under salinity 16 psu, while the content of immunoglobulin was significantly lower than that of the control group in freshwater. Gut microbiota diversity in the fish fed B. subtilis E221 was higher than in the control group at 16 psu. PICRUSt analysis showed that the pathways of the renin-angiotensin system, inorganic ion transport and metabolism in tilapia fed B. subtilis E221 at 16 psu were significantly upregulated than those in the control fish in freshwater. These findings suggest that B. subtilis E221 could be a potential dietary probiotic for improving the growth and health status of O. niloticus under hypersaline stress by modulating its intestinal microbiota.