Screening and identification of Bacillus velezensis FLU-1 from the intestinal tract of largemouth bass and its use as a feed additive

Abstract

Probiotics have been widely used in aquaculture and may serve as a potential alternative to antibiotics. Host-derived probiotics are widely used in aquaculture because they are able to adapt to the host intestinal environment more easily than other probiotics. This study was conducted to evaluate the probiotic potential of the bacteria isolated from the gut of largemouth bass. The actions of dietary supplementation with B. velezensis FLU-1 were investigated in the largemouth bass with respect to growth, the morphology of the intestine, digestive and immune functions, and antioxidant capacity, as well as intestinal microbiota. The results revealed that B. velezensis FLU-1 exhibited promising probiotic traits, including extracellular enzyme production, ability to withstand acidic conditions, high bile salt concentration, and elevated temperatures. Furthermore, it demonstrated high hydrophobicity and auto-aggregation cability, alongside being free from antibiotic resistance and displaying a non-hemolytic nature. A diet with host–derived B. velezensis FLU-1 supplementation improved the growth performance of the fish. It also increased the length of the intestinal villi and tight junction gene expression levels, including claudin-2, occludin, and ZO-1. Host–derived B. velezensis FLU-1 supplementation enhanced the activities of protease, α-amylase, lipase, alkaline phosphatase, acid phosphatase, lysozyme, catalase, glutathione peroxidase, decreased the level of MDA, increased the level of the anti-inflammatory cytokine TGF-β, and decreased the level of the pro-inflammatory cytokine TNF-α. Furthermore, B. velezensis FLU-1 increased the levels of several probiotics, including Lactobacillus and Lactococcus, and bacteria that produce short-chain fatty acids, including Faecalibacterium, Bacteroides, and Clostridium. The results in vivo show that adding B. velezensis FLU-1 to the feed could reduce the mortality of largemouth bass after infection with A. hydrophila, as well as reduce the bacterial load in the spleen.The results indicated that further study is warranted concerning the use of B. velezensis FLU-1 combined with sodium gluconate as a diet supplement in other economically viable fish.