Probiotic potential and immunomodulatory effects of a thermophilic Bacillus sp. isolated from a hydrothermal vent

Antibiotic resistance has intensified the need for alternatives, and probiotics offer a natural strategy by enhancing host immunity. The present study investigated the probiotic potential of Bacillus sp. 475B, a strain isolated from a high-temperature hydrothermal vent. In vitro assessments included its safety profile (cytotoxicity, hemolytic activity, and antibiotic susceptibility), tolerance to gastrointestinal and physicochemical stressors, antimicrobial activity, and gut adherence capacity. Additionally, the immunomodulatory effects of Bacillus sp. 475B were evaluated in mice supplemented daily (1 × 10⁸ CFU). Splenocytes were isolated on days 5 and 10, challenged ex vivo with Escherichia coli, and analyzed for immunological assays. Serum and fecal IgA and IgG levels were also quantified. Bacillus sp. 475B was non-cytotoxic, non-hemolytic, and susceptible to most tested antibiotics. The strain tolerated low pH, bile salts, high salinity, and high temperatures, as well as the capacity to adhere to the jejunum of healthy mice determined by fluorescence microscopy. Supernatants from cultures at 30 and 50 °C inhibited the growth of methicillin-resistant Staphylococcus aureus (MRSA). In vivo enhanced immune responses as indicated by increased nitric oxide production and myeloperoxidase activity. Antibody levels were not affected by supplementation with the strain. These findings highlight the potential of Bacillus sp. 475B is a robust and functional probiotic candidate for future applications in health and biotechnology.