Exploration of bioactive secondary metabolites from probiotic <i>Bacillus paramycoides</i> strains isolated from <i>Channa</i> species: isolation, analysis and antimicrobial profiling against pathogenic <i>Aeromona</i> s <i>hydrophila</i> (MTCC 12301

Bacterial infections remain a major challenge in aquaculture, and the extensive use of antibiotics has contributed to the development of antimicrobial-resistant bacteria. This situation has intensified the search for natural and sustainable antibacterial agents that can effectively control fish diseases while reducing reliance on conventional antibiotics. In this study, extracellular metabolites (ECMs) and whole-cell metabolites (WCMs) produced by two Bacillus paramycoides strains (OQ300496 and OQ303631) were characterized using Gas Chromatography – Mass Spectrometry (GC – MS) and evaluated for their antibacterial activity against Aeromonas hydrophila, a common fish pathogen. GCMS identified seventy-eight compounds in the extracts, with the most abundant compounds being Androst-8-en-3-ol, 4,4,14-α-trimethyl-17-(2-bromo-1-methylethyl)- (37.34%); Oxalic acid monoamide, N-(3,4-dimethylphenyl)-isohexyl ester (65.6%); 2,2-Dimethyl-6-methylene-1-[3,5-dihydroxy-1-pentenyl] cyclohexan-1-perhydrol (12.37%); Labda-8(20),14-diene-13,19-diol (20.4%); and Abietaldehyde (19.22%). The crude extracts exhibited moderate antibacterial activity against A. hydrophila, producing inhibition zones ranging from 8.0–9.6 mm, with the WCM of strain OQ300496 showing the greatest inhibitory activity. These findings indicate that B. paramycoides strains OQ300496 and OQ303631 produce bioactive metabolites with potential as sustainable antibacterial agents for controlling fish bacterial infections.