LAUSR

Abstract Marine sponges harbor diversified bacterial symbionts many of which produce a wide array of bioactive compounds including antibiotics. The sponge-associated bacteria of Saint Martin's Island area of the Bay of Bengal, Bangladesh is largely unexplored. This study, therefore, aimed to isolate and identify the bacteria from sponges of the Saint Martin's Island area for assessing the diversity of these sponge-associated bacteria, to determine the in vitro inhibitory effects of their extracellular products (ECPs) against fish pathogens, and to assess the prophylactic efficacy of ECPs of sponge-associated bacteria against Motile Aeromonas Septicemia (MAS) in Labeo rohita. A total of 645 bacterial isolates were isolated from nine sponges collected from different locations of Saint Martin's Island. Among these, 15 bacterial genera were identified based on their colony, morphological, physiological, and biochemical characteristics. Sixty representative isolates were identified up to species level through 16S rRNA gene sequence analyses where 31 different bacterial species were explored. Among these, Vibrio spp. was the most dominant bacteria followed by Bacillus spp., Burkholderia spp., and Oceanobacillus spp. All the bacterial isolates were screened against eight different fish and shrimp pathogens through in vitro agar well diffusion assays to identify potential antibiotic-producing bacteria. Bacillus subtilis strains WS1A and YBS29 were found to inhibit the fish pathogenic Aeromonas veronii strains. Fingerlings of L. rohita were fed the ethyl acetate extracts of ECPs of WS1A and YBS29 incorporated commercial feeds for 30 days. Then, fish were artificially challenged with A. veronii strains B55 and Aero1. Control group fish were fed the basal diet. Interestingly, WS1A and YBS29 treated fish showed no disease symptoms and mortality whereas severe disease symptoms with 100% mortality were found in the challenged control. The whole-genome sequence of B. subtilis strain WS1A was analyzed that decoded a number of orthologs to intrinsic genes of antimicrobial peptides. Finally, these results indicate that Bacillus subtilis strains WS1A and YBS29 could be promising for controlling MAS in L. rohita.