Strain-specific effects of fish originated Bacillus velezensis on growth, gut health, and disease resistance of zebrafish.

The function and safety of probiotics are specific at the strain level. Accurate identification and evaluation of probiotics is very important for the development of the industry. In this work, five strains of Bacillus velezensis isolated from large yellow croaker (Larimichthys crocea) were classified into two genotypes by random amplification polymorphic DNA (RAPD) strain typing.The representative strain T1 had a strong ability to inhibit pathogenic bacteria, and T23 has a strong ability to secrete digestive enzymes in vitro. The growth-promoting and disease-resistance abilities of T1 and T23 in zebrafish were then compared. The results illustrated that T23 significantly promoted the weight gain of zebrafish. T23 significantly enhanced the activity of lipase, and reduced the feeding metabolic energy to reduce the total metabolic energy. Although the T1 group had higher protease activity, but there were no obvious changes in total metabolic energy, and its growth-promoting effect was not obvious. In terms of intestinal morphology, both strains significantly increased the length of intestinal villi, but T23 had a stronger ability to maintain the thickness of the muscle layer, also significantly reduced serum lipopolysaccharide (LPS) level. qPCR detection of genes related to innate immunity showed that T1 significantly down-regulated the expression of TNF-α and IL-1β, and significantly up-regulated the expression of hepcidin. T23 significantly reduced the expression of TNF-α, and significantly improved the expression of hepcidin. In the A. versonii challenge test both T1 and T23 improved the survival rate of zebrafish, and T23 showed better capacity. 16S rRNA sequencing of intestinal microbiota showed that T1 and T23 could change the structure of the microbiota. The intestinal microbiota induced by the two supplement groups both affected the expression levels of growth-related genes and innate immunity-related genes in germ-free zebrafish (GF-zebrafish). When GF-zebrafish were directly immersed with T1 and T23, the results illustrated no significant changes in the expression of growth-related genes, but T1 significantly increased the expression of TNF-α, while T23 significantly affected the expression of IL-1β and IL-10. Comparative genomic analysis of T1 and T23 implicated that T23 strain may be more comprehensive and complex in function. Together, our results showed strain specific property of probiotic effects, and revealed potential microbiota-associated mechanism of the effect of B. velezensis strains on growth and immunity of fish.