Abstract
Motile Aeromonas Septicemia is an important bacterial disease affecting freshwater aquaculture and is commonly associated with infection by Aeromonas hydrophila in cultured fish. The present study evaluated soil-derived Streptomyces spp. for their antibacterial and disease-management potential against Aeromonas infection in Catla catla. Actinomycetes were isolated from soil samples collected from different habitats and screened on selective medium. Presumptive Streptomyces isolates were characterised using morphological, biochemical, and molecular approaches. Secondary metabolites were produced under submerged fermentation and extracted using ethyl acetate. The crude extract was evaluated for antibacterial activity against the target bacterial pathogen using the agar well diffusion method, and the minimum inhibitory concentration was determined by a resazurin-based microdilution assay. Among the isolates recovered, one Streptomyces isolate showed notable antibacterial activity. The extract produced concentration-dependent inhibition, with a maximum inhibition zone of 24.6 ± 1.8 mm at 10 mg/mL. The minimum inhibitory concentration was 62.5 µg/mL. In the in vivo challenge study, fish receiving Streptomyces-supplemented feed showed improved survival after bacterial challenge, with 72% survival in the treated group. The observationsindicate that soil-derived Streptomyces spp. may provide antibacterial metabolites with potential applications in aquaculture disease management. However, further characterisation of the active compounds, confirmation of pathogen identity, optimisation of dosage, and validation under controlled and field conditions are required before practical application can be recommended.