Probiotics are living microorganisms that confer positive effects on the health of the digestive system when incorporated with feed in a defined dosage and help to stimulate the immune response in fish against virulent fish bacterial pathogens. Therefore, focusing on the gastrointestinal health of fish is more essential. This study aimed to determine the antagonistic activity of bacteria isolated from the gut of freshwater fish  Dawkinsia filamentosa  and the immune response in Channa striatus challenged with Aeromonas hydrophila. In total, 15 bacterial strains with different colony morphology were isolated; further screened for antibacterial activity using the agar overlaying and well diffusion method against Aeromonas hydrophila, Pseudomonas aeruginosa, Vibrio cholerae, and Staphylococcus aureus. In the preliminary screening, four isolates showed considerable activity. Among them, DF12 was found to have prominent antagonistic activity against fish pathogens. In the agar overlaying method, the strain DF12 showed the highest inhibitory zone of 8 mm against Pseudomonas aeruginosa, Staphylococcus aureus, Aeromonas hydrophila and 6 mm against Vibrio cholerae. In the well diffusion method, the isolate DF12 showed the highest activity against S. aureus (10 mm), V. cholerae, and P. aeruginosa with 8 mm each, the active bacterial strains were identified up to genus level as Enterococcus sp. (DF6), Lactobacillus sp. (DF8), and Bacillus sp. (DF12) based on morphological and biochemical characterization. The isolate Bacillus sp. possesses a high ability to tolerate low to high pH range of 4 to 8 and the bile salt concentration of 0.0, 0.15, and 0.30%, This present investigation substantiates that the fish gut can be explored as a promising source of antagonistic bacteria and develop an immune response that produces a wide spectrum of bioactivity. These probiotic bacteria have warranted much research interest because of their broad specificity on antibacterial potential and even immunostimulant effects.

Keywords:  Dawkinsia filamentosa, Gut probiotics, Antagonistic activity, Immune response, Channa striatus