In vitro selection, assessment, and identification of potential probiotic features of the Bacillus strains isolated from the intestine of Oreochromis niloticus

In this study, we aimed to explore the probiotic potential of bacterial species isolated from the digestive tract of Nile tilapia ( Oreochromis niloticus ). Through morphological, biochemical, 16S rDNA sequencing, and evolutionary relationship assessments, we identified the isolated species as Bacillus amyloliquefaciens AV5 (OR647358), Bacillus subtilis AV7 (LC781790), and Bacillus velezensis AV50 (OR647359), designated as AV5, AV7, and AV50 strains, respectively. We evaluated their probiotic potential, including tolerance to high bile salt concentrations, low pH levels, and high temperatures, as well as their adhesion abilities (auto-aggregation and cell-surface hydrophobicity), antimicrobial activity, biosafety, compatibility, hemolytic activity, and antibiotic susceptibility. AV5, AV7, and AV50 strains exhibited γ-hemolytic activity and resistance to low pH (1) and high bile salt concentrations (0.5%). They demonstrated higher viability after exposure to elevated temperatures (80, 90, and 100°C) and increased cell surface hydrophobicity and auto-aggregation. These strains showed positive compatibility, indicating their potential for multispecies use. Strains were susceptible to various antibiotics, showing susceptibility to 17 out of 20 tested antibiotics, and displayed significant antimicrobial activity against V. alginolyticus , V. harveyi , S. iniae , and S. agalactiae . Therefore, AV5, AV7, and AV50 are promising probiotic candidates for enhancing the growth and health of aquatic animals, particularly the Nile tilapia.