Antibacterial, Cytotoxic, and Cellular Mechanisms of Green Synthesized Silver Nanoparticles against Some Cariogenic Bacteria (Streptococcus mutans and Actinomyces viscosus)

Background. The present study focused on the green synthesis of silver nanoparticles (AgNPs) using the Astragalus spinosus Forssk. aqueous extract. In addition, we evaluated the antibacterial activity of AgNPs as well as some cellular mechanisms against Actinomyces viscosus and Streptococcus mutans as the most causative agents of tooth decay. Methods. In this study, AgNPs were green synthesized by the precipitation method based on the reduction of silver ions (AgNO3) by A. spinosus extract. Antibacterial effects of the green synthesized AgNPs were performed by measuring the minimum inhibitory concentration (MIC) and minimum bactericidal concentrations (MBC) through micro broth dilution method. In addition, we evaluated the reactive oxygen species (ROS) production, nucleic acid leakage, and protein leakage as the main antibacterial mechanisms of the green synthesized AgNPs against A. viscosus and S. mutans. The cytotoxicity effects of AgNPs against on human normal (NOF18 cells) and oral cancer (SCC4 cells) cell lines were also evaluated using MTT assay. Results. The green synthesized AgNPs have a spherical shape and are relatively uniform in size in the range of 30-40 nm. The MIC values for S. mutans and A. viscosus of the green synthesized AgNPs were 10.6 and 13.3 mu g/ml, respectively, whereas the MBC values for S. mutans and A. viscosus of the green synthesized AgNPs were 21.3 and 26.6 mu g/ml. The findings exhibited that ROS production, nucleic acid leakage, and protein leakage were increased after treatment of A. viscosus and S. mutans by the green synthesized AgNPs. The results demonstrated that the 50% inhibitory concertation (IC50) values of AgNPs on NOF18 and SCC4 cells were 93.3 mu g/ml and 41.2 mu g/ml, respectively. Conclusion. Overall, the results of this study showed that A. spinosus extract has a good ability to produce silver nanoparticles. The AgNPs produced have significant antibacterial effects against some tooth decay bacteria. Our results also revealed that the green synthesized AgNPs are more cytotoxic against cancerous cell line than normal cell line. Further in vivo studies are required to investigate the side effects and to evaluate the effectiveness of these bacteria.