Isolation, Characterization, and Assessment of Potential Probiotic Bacteria in the Gastrointestinal Tract of Clarias magur (Hamilton, 1822) for Aquaculture Applications

Probiotic bacteria play an important role in boosting the immune system, aiding enzymatic digestion, maintaining intestinal balance, and inhibiting the growth of harmful microbes in freshwater fish. Probiotics isolated from the intestines of the same host species are generally more effective for colonization than those obtained from unrelated sources. This study evaluated the probiotic potential of gut-derived bacteria from Clarias magur. A total of 160 biological samples were collected from multiple locations across four districts in Assam, India. The tested bacterial isolates exhibited significantly different survival rates under various stress conditions. Sixteen isolates survived simulated gastrointestinal conditions (pH 2), with survival rates ranging from 60.90 to 96.30%. In bile tolerance tests, 13 isolates survived in 0.3% bile salt, showing varying tolerance levels. After one hour of exposure to 1% lysozyme (100mg/ L), all 13 isolates demonstrated good survival, with rates ranging from 63.10 to 95.50%. In the presence of 20mM hydrogen peroxide (H₂O₂), 10 isolates remained viable, with survival rates between 43.60 and 72.00%. The 13 isolates exhibited auto-aggregation abilities ranging from 45.13 to 78.70% after 5 hours of incubation, while cell surface hydrophobicity in xylene ranged from 56.55 to 92.00%. Eight isolates displayed varying degrees of antagonistic activity against four fish pathogens: Aeromonas hydrophila, Aeromonas jandaei, Pseudomonas aeruginosa, and Staphylococcus aureus. All lactic acid bacteria (LAB) isolates showed different susceptibility and resistance patterns to clinically relevant antibiotics. Importantly, all strains were γ-hemolytic, exhibiting no hemolytic activity on sheep blood agar.
Molecular sequencing and phylogenetic analysis identified eight LAB isolates as Lactobacillus reuteri, Limosilactobacillus reuteri, Enterococcus faecalis, and Enterococcus mundtii. The findings highlight the probiotic potential of LAB isolated from freshwater fish and their possible application as biocontrol agents in aquaculture.