Extracellular Polysaccharide From Marine Streptomyces sp. KAR3: Structural Characterization, Antibacterial, and Anticancer Potential.
Marine environments are an abundant source of sustainable polymers with considerable biomedical potential. In this study, a marine Streptomyces sp. strain KAR3 was isolated from the Gulf of Mannar. Extracellular polysaccharide (EPS) was isolated, characterized, and evaluated for its antibacterial, antibiofilm, and anticancer activity against MDA-MB-231 cells. Strain identification using biochemical characterization and 16S rRNA gene sequencing (Accession: PP330035) showed 100% similarity to Streptomyces geysiriensis, and phylogenetic analysis supported its affiliation with the S. geysiriensis clad. S. geysiriensis strain KAR3 produced 4.4 g/L of EPS in the media after 7 days. EPS production was optimized in several culture media and structurally characterized by UV spectroscopy, Fourier transform infrared (FTIR), 1H and 13C nuclear magnetic resonance (NMR), and GC-MS. FTIR analysis revealed C-H and C = O stretching and glycosidic linkages. 1H and 13C NMR shifts aligned with sugar moieties, indicating the EPS is a complex polysaccharide. GC-MS showed glucose as the major monosaccharide in the EPS. The isolated EPS exhibited dose-dependent inhibition, with the largest zone of inhibition observed at 100 µg/mL. The EPS exhibited notable antibacterial and biofilm inhibitory activity against Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, and Enterococcus faecalis in a concentration-dependent manner. The minimum inhibitory concentration (MIC) calculated for the most susceptible strains was at 100 µg/mL. The IC50 value of EPS was found to be 25.44 µg/mL, indicating a strong cytotoxic effect against MDA-MB-231 cancer cells. Together, these findings demonstrate the broad-spectrum antibacterial and anticancer activity of marine EPS from S. geysiriensis and its potential as a natural source in the search for new antimicrobial compounds.