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The world faces a challenge with the pervasion of multidrug-resistant bacteria that encourages scientists to develop and discover alternative, ecofriendly, and easy-to-produce new antibacterial agents. Our work is part of the greater effort of scientists around the world to achieve this goal by the biological synthesis of silver nanoparticles using cyanobacterial extracellular and intracellular components as nonchemical reducing agents. Two Egyptian cyanobacteria were isolated and identified according to 16S rRNA gene sequencing as Phormidium ambiguum and a novel species Desertifilum tharense. The sequences were deposited with accession numbers MW762709 and MW762710 for Desertifilum tharense and Phormidium ambiguum, respectively, in the GenBank. The results of UV-Vis analysis showed promising extracellular Ag-NPs synthesis by Desertifilum tharense and Phormidium ambiguum under light conditions. Therefore, these Ag-NPs were characterized and evaluated for antibacterial and antioxidant activity. TEM and SEM analyses revealed the spherical crystals with face-centered cubic structures and size range of 6.24–11.4 nm and 6.46–12.2 nm for Ag-NPs of Desertifilum tharense and Phormidium ambiguum, respectively. XRD and EDX results confirmed the successful synthesis of Ag-NPs in their oxide form or chloride form. The FTIR spectrum data confirmed the presence of hydroxyl and amide groups. Desertifilum tharense Ag-NPs displayed the largest inhibition zone that ranged from 9 mm against Micrococcus luteus ATCC 10240 to 25 mm against methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300. For Phormidium ambiguum Ag-NPs, the inhibition zone diameter was in the range of 9 mm to 18 mm. The biosynthesized Ag-NPs significantly inhibited the growth of medically important resistance-pathogenic Gram-positive and Gram-negative bacteria. The Ag-NPs of Phormidium ambiguum exhibited the highest scavenging activity of 48.7% when compared with that of Desertifilum tharense, which displayed 43.753%.

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The association between the oral microbiota and oral diseases is well established. Various antimicrobial agents including antibiotics are commercially available against oral pathogenic bacteria. For the reasons of antibiotic resistance, their adverse effects and financial considerations in the developing countries, there is a need for alternate preventive and curative treatment options that are also safe, effective and economical. Traditional medicines have been used since ancient times for the treatment of oral diseases including dental caries, periodontal diseases that affect the majority of the population and can affect a person's overall health. Natural phytochemicals are certain organic components isolated from plants and some of these extracts are considered to be beneficial to health. They serve as antioxidants, enhance immune response,provide protection against oral cancer and other diseases and also repair DNA damage caused by smoking and other toxic exposure, and detoxify carcinogens. The natural products derived from medicinal plants have proven to be an abundant source of biologically active compounds, many of which have been the basis for the development of new lead chemicals for pharmaceuticals.They are considered to be good alternatives to synthetic chemicals. This article presents a review of natural alternatives derived from plants and plant products that can serve as a prevention and treatment option against cariogenic bacteria.