Ajani O.A. , Osho  E.F. , Onada  O.A.

University of Ibadan, Nigeria
Author    Correspondence author
International Journal of Aquaculture, 2026, Vol. 16, No. 4   
Received: 11 May, 2026    Accepted: 30 Jun., 2026    Published: 27 Jul., 2026

This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The escalating global aquaculture production necessitates sustainable alternatives to traditional antibiotics due to concerns regarding antimicrobial resistance and environmental contamination. This study investigated the probiotic potential and molecular identity of autochthonous bacterial isolates from the gastrointestinal tract of Oreochromis niloticus (Nile tilapia). Microbial populations from the upper and lower intestinal segments were enumerated and characterized using morphological, biochemical, and molecular methods. Results revealed significant spatial variation in bacterial distribution along the gastrointestinal tract, with total bacterial abundance significantly higher (p < 0.05) in the lower intestine than in the upper intestine. Among the bacterial groups identified, Bacillus spp. showed strong probiotic potential due to their high abundance, spore-forming capacity, and tolerance to gastrointestinal stress conditions. The isolates were Gram-positive, non-hemolytic, and demonstrated strong survival under acidic conditions (pH 2.5~7) and bile salt concentrations up to 1%. Antimicrobial activity assays revealed significant inhibition of pathogenic bacteria including Escherichia coli, Salmonella typhi, and Staphylococcus aureus, with inhibition zones reaching 34 mm. Molecular characterization using 16S rRNA gene sequencing confirmed the isolates as Bacillus clausii. Phylogenetic analysis showed high genetic similarity with previously reported Bacillus clausii strains in the NCBI database. These findings suggest that autochthonous Bacillus clausii isolated from O. niloticus possess desirable probiotic characteristics and may serve as promising candidates for functional probiotic applications in sustainable aquaculture.

Probiotics; Bacillus clausii; Nile tilapia; gut microbiota; antimicrobial activity; aquaculture biotechnology