Isolation and screening of probiotics candidates from the intestinal tracts of Nile tilapia (Oreochromis niloticus)

Probiotics play a vital role in aquaculture by maintaining digestive tract balance, enhancing enzymatic digestion, synthesizing essential biomolecules, increasing nutrient availability, strengthening intestinal function, detoxifying harmful compounds, modulating immune responses, and improving resistance to pathogens in fish farming. Notably, probiotics derived from the gastrointestinal tract of the same host species exhibit higher colonization efficiency and provide greater benefits compared to those from external sources. Thus, this study aimed to isolate, identify, and evaluate the probiotic potential of bacteria obtained from the digestive tracts of Nile tilapia (Oreochromis niloticus) cultured in a rice-fish farming system. The research followed a systematic approach, including bacterial isolation, phenotypic and molecular identification, and an assessment of probiotic properties. A total of 27 bacterial isolates were successfully obtained, and five of which had a capacity to produce digestive enzyme (protease), exhibited strong antimicrobial activity against Aeromonas hydrophila. Based on their 16S rRNA gene sequences, two isolates were identified as Kurthia gibsonii (99.43% and 100% similarity), one isolate as Bacillus thuringiensis (91.41% similarity), one isolate as Bacillus cereus (85.01% similarity), and the other isolate as Lactococcus lactis (100% similarity). Further analysis confirmed that all isolates were non-pathogenic and susceptible to four antibiotics (Tetracycline, Amoxicillin, Chloramphenicol, and Gentamicin). Additionally, all isolates demonstrated high tolerance to variations in pH and temperature. Among the five isolates, L. lactis exhibited the strongest pathogen inhibition and the highest tolerance to environmental fluctuations, suggesting its superior probiotic potential. Overall, these findings indicate that the isolated bacteria, particularly L. lactis, hold promise as probiotic agents and dietary supplements for improving fish health and enhancing aquaculture sustainability.