The Antimicrobial and Cytotoxic Activity of Green-Synthesized Silver Nanoparticle Extract of Marine Bacteria and Their 16S rRNA Identification and Characterization.

Marine bacteria represent a substantial underexplored reservoir of bioactive metabolites with significant biomedical potential. Through 16S rRNA gene sequencing, this study identified four bacterial isolates associated with the marine sponge Callyspongia siphonella. They were Gammaproteobacterium, Idiomarina sp., Halomonas sp., and Oceanobacillus kapialis. The isolates were used to synthesize silver nanoparticles (AgNPs) in an environmentally friendly way. Subsequently, the antibacterial and cytotoxic activities of these nanoparticles were evaluated. All biosynthesized AgNPs were very small (7.28-13.8 nm) and effective against common pathogens, with minimum inhibitory concentrations between 8 and 16 μg/mL. The nanoparticles exhibited no detectable inhibitory activity against Candida albicans, which means that they only worked against bacteria. Cytotoxicity assays showed that the four AgNP preparations had the same IC50 values (42-44 μg/mL) and that the cytotoxic activity against rat cancer cells increased with concentration. This indicates that the physicochemical characteristics of the nanoparticles, rather than their bacterial source, predominantly affected cytotoxic efficacy. Gram staining and microscopy-based phenotypic characterization exhibited significant concordance with molecular identification, thereby reinforcing taxonomic classifications. These findings collectively highlight marine sponge-associated bacteria as potential bioreactors for the sustainable synthesis of bioactive AgNPs with selective antimicrobial activity and moderate cytotoxic properties.