Functional probiotic attributes of Bacillus subtilis ULB16, an endophyte isolated from marine macroalga, Ulva lactuca

Marine-derived probiotic bacteria have emerged as promising alternatives to antibiotics in sustainable aquaculture owing to their resilience, metabolic versatility, and ability to produce beneficial bioactive compounds. In the present study, the probiotic and functional potential of Bacilllus subtilis ULB16, a marine endophytic bacterium isolated from Ulva lactuca, was comprehensively evaluated. The safety assessment revealed susceptibility to several clinically relevant antibiotics, while no extracellular DNase activity was detected, supporting its non-virulent nature. The strain exhibited tolerance to gastrointestinal-like stress conditions, including acidic pH, bile salts, and simulated gastric juice, indicating its ability to survive gastrointestinal transit. Functional probiotic characterization demonstrated cell surface hydrophobicity, auto-aggregation, and co-aggregation with representative enteric pathogens (Escherichia coli, Salmonella enterica, and Klebsiella pneumoniae) as well as aquaculture-associated pathogens (Aeromonas hydrophila and Vibrio parahaemolyticus). B. subtilis ULB16 also exhibited biofilm-forming ability and showed higher adhesion to HT-29 intestinal epithelial cells than the commercial probiotic strain Alkalihalobacillus clausii. Notably, the isolate exhibited β-galactosidase activity of 0.0565 U/mL, a relatively uncommon feature among Bacillus-based probiotics, suggesting additional nutritional and functional benefits associated with lactose metabolism. Collectively, the findings demonstrate that B. subtilis ULB16 possesses a combination of desirable probiotic traits, including stress tolerance, pathogen interaction capability, epithelial adhesion, and functional enzyme activity. These multifunctional properties highlight its potential as a promising marine-derived probiotic candidate for sustainable biotechnological applications.