Background: The antimicrobial activity of silver nanoparticles has been investigated in medical fields in recent years, but there are few studies regarding its effect on oral microorganisms. The aim of the present study was to evaluate the in vitro antimicrobial and toxicity properties of nanosilver against two dental plaque microorganisms  and Human Gin- gival Fibroblast (HGF) cell line.

Methods: Antibacterial effects of nanosilver colloidal solution were de-

termined by minimal inhibitory concentration (MIC) and minimal bacte- ricidal   concentration   (MBC)   using  microdilution   method.   Standard strains of Streptococcus sanguis and Actinomyces viscosus were used. For toxicity assessment,  MTT  and LDH  tests were performed  under  con- trolled conditions. Different concentrations of nanosilver were prepared and their toxic effects  on HGF were determined  after 24, 48 and 72 hours.

Results: The MIC of nanosilver solution for S. sanguis and A. viscosus

were 16 and 4 µ g/ml, respectively. The MBC of nanosilver was 64 µ g/ml for S. sanguis and 16 µ g/ml for A. viscosus. MTT results showed that after 24 hours the concentrations of ≥ 0.5 µ g/ml of nanosilver solution affected cell viability when compared with control group. After 48 and 72 hours only the concentration of  ≥ 5 µ g/ml showed significant effect on cultured cell viability. LDH release test demonstrated toxic effect only after 48, 72 hours by 20 and 50 µ g/ml of nanosilver.

Conclusion: The results demonstrated that beside its antibacterial activity

against S. sanguis and A. viscosus, nanosilver mediated a concentration and time dependent cytotoxicity on HGF.