Untapped Antimicrobial Potential: Exploration of Bioactive Compounds from Bacterial and Fungal Symbionts of Marine Bivalves

Bivalves harbour diverse microbial symbionts capable of producing bioactive compounds with antimicrobial potential. This study explores the antibacterial activity and secondary metabolite content of the bivalve Polymesoda erosa and its microbial symbionts. It has the following objectives: (a) identify the bivalve species and isolate bacterial and fungal symbionts; (b) analyse secondary metabolites using phytochemical tests and gas chromatographymass spectrometry (GCMS); (c) evaluate the antibacterial activity of tissue extracts and symbiont isolates against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Cutibacterium acnes; and (d) compare their bioactivity to identify the most promising antimicrobial source. Bacterial and fungal symbionts were isolated and identified using molecular and morphological methods. Antibacterial activity was evaluated in vitro, and chemical profiles were analysed using GCMS. The bivalve was identified as Polymesoda erosa, with Staphylococcus arlettae as its bacterial symbiont and Penicillium sp. as the fungal symbiont. The host tissue extract exhibited weak antibacterial activity, with a maximum inhibition zone of 0.825 mm against E. coli, and contained primary metabolites such as palmitic acid, stearic acid, cholesterol and stigmasterol. In contrast, S. arlettae exhibited broader activity, inhibiting both E. coli and S. aureus, and produced amylase and lipase enzymes. The fungal symbiont Talaromyces cf. Penicillium sp. demonstrated the most potent antibacterial effect, with an inhibition zone of 20.15 mm against S. aureus. These results indicate that the microbial symbionts of P. erosa show superior and broader-spectrum antimicrobial activity compared with the host. Both bacterial and fungal symbionts are promising sources for developing novel marine-derived antimicrobial agents.