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Probiotics are living microorganisms that have positive effects on the health of the digestive system when acquired in a defined dosage and helps to stimulate the immune response in fish against deadly pathogens. Therefore, focusing on the maintenance of gastrointestinal health is more essential. This study aimed to determine the antagonistic activity of bacteria isolated from the gut of freshwater fish Etroplus maculatus. In the present study, we isolated 20 bacterial isolates with different colony morphology; further screened for antibacterial activity using the cross streaking and disk diffusion method against Aeromonas hydrophila, Pseudomonas aeruginosa, Vibrio cholerae, and Staphylococcus aureus. In the preliminary screening isolates EF1 and EM9 were found to have prominent antagonistic activity. In the cross-streaking method, the isolates EM9 showed the highest level of inhibitory (85%) against Staphylococcus aureus; a moderate level of activity in EM9 against Aeromonas hydrophila and the lowest level of inhibition in EF1 and EM9 against Pseudomonas aeruginosa and Vibrio cholerae. The bacterial metabolites of active bacterial isolates (EF1 and EM9) were screened for their bioactive potential. Ethyl acetate extract of EM9 isolate has been shown to have predominant activity against fish pathogens at 100 µg/disc concentration with a zone of inhibition of 14mm against Staphylococcus aureus followed by EF1 where the zone of inhibition was highest with 12mm. The active bacterial isolates were identified as Escherichia sp and Bacillus sp based on morphological and biochemical characteristics. This investigation clearly shows that the fish gut can be explored as a source of antagonistic bacteria which produces antimicrobial compounds of wide spectrum of bioactivity. These bacterial secondary metabolites have warranted much research interest because of their biochemical diversity, broad specificity on antimicrobial, and even anti-tumoural or wound-healing effects.
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