The regulatory effects of Bacillus circulans on the growth performance, immune responses, and intestine microbiota of Litopenaeus vannamei

Probiotics have become a widely recognized method for controlling diseases in aquaculture. The purpose of this study was to investigate the effects of spraying Bacillus circulans into the water on the growth, immune response, and gut microbiota of Litopenaeus vannamei , under the condition of feeding the same diet. In contrast to traditional feed supplementation approaches, the present work investigated the efficacy of B. circulans when introduced directly into aquaculture water systems as free-living suspensions. Four randomly chosen groups of 240 healthy shrimp (3.00 ± 0.30 cm) were administered B. circulans at different concentrations (C: 0; L: 1.00 × 10⁵ CFU/mL; M: 1.00 × 10⁶ CFU/mL; H: 1.00 × 10⁷ CFU/mL). The final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) of juvenile Litopenaeus vannamei in the probiotic-treated groups significantly improved after 42 days of supplementation, especially in the 10 6 CFU/mL group. The digestive enzymes and antioxidant enzyme activity of the treatment groups also increased noticeably. Additionally, shrimp that received a B. circulans supplement displayed enhanced immunity, which was reflected by the upregulation of immune-related genes (e.g., lectin , crustin ). The groups sprayed with B. circulans exhibited greater intestinal microbial diversity and richness than the groups not sprayed with B. circulans , according to alpha diversity indices. According to beta diversity analysis, the bacterial communities in the B. circulans treatment groups differed from those in the non-sprayed groups but were similar to one another. After treatment with B. circulans , the relative abundance of potential pathogenic genera, such as Vibrio and Tenacibaculum , was significantly reduced. In summary, B. circulans regulated the intestinal microbiota of L. vannamei , enhanced digestion and immunity, and significantly boosted growth. Based on these results, we recommend the use of a B. circulans spray application at a concentration of 10 6 CFU/mL to optimize shrimp growth and health. • Isolation of B. circulans from L. vannamei intestine, showing potential as a probiotic for aquaculture. • B. circulans positively affects the growth, immunity, and intestinal microbiota of L. vannamei . • The optimum concentration of B. circulans for L. vannamei was 10 6 CFU/mL.