Probiotic potential of Bacillus velezensis STPB10 sourced from the gut microbiota of a hillstream fish Schizothorax richardsonii (Gray, 1832) for aquaculture applications

The study focused on the evaluation of the probiotic potency of Bacillus velenzensis STPB10, isolated from the digestive tract of the hillstream fish Snow trout (Schizothorax richardsonii). Ten B. velezensis strains were identified through colony morphology, biochemical analysis, and 16 S rRNA methods. The representative laboratory strain STPB10, was subjected to various in vitro assessments including tolerance to pH, bile salt resistance, NaCl, temperature, biofilm, endospore formation, antibiotic susceptibility and antagonistic activity to determine its potential as a probiotic strain. B. velezensis STPB10 exhibited positive results for catalase, glucose, arginine, lysine, and endospore tests, while it yielded negative results for oxidative/fermentative, DNase, and urease tests. This test isolate was γ-hemolytic in nature and demonstrated growth across a temperature range of 15 to 35 °C. Moreover, it exhibited survival rates of 31, 39.7, 47, 63, and 75% at pH values of 2, 2.5, 3, 3.5, and 4, respectively, after 6 h of incubation. At a bile salt concentration of 0.3%, the bacterium exhibited a survivability of more than 99%. A strong biofilm production by B. velezensis STPB10 was detected in tryptone soya broth enriched with 0.45% glucose (p < 0.05). It also exhibited significantly greater adhesion to intestinal mucus (63.67%). Following exposure of the intestinal mucus to B. velezensis STPB10, the adhesion of A. hydrophila, A. veronii, (A) salmonicida and V. anguillarum to the mucus was notably reduced. It was susceptible to several antibiotics and produced an antagonistic effect against pathogenic bacteria Aeromonas salmonicida, Aeromonas veronii, Vibrio anguillarum, and Aeromonas hydrophila isolated from diseased fish. The pathogenicity of (B) velezensis STPB10 through intraperitoneal injection and immersion challenge at cell concentrations of 108 and 109 CFU mL-1 revealed that the strain did not produce any pathogenic risk to common carp. These findings highlight the resilience and adaptability of B. velezensis STPB10 as a candidate probiotic in aquaculture.