Antibacterial effect of silver and fluoride compounds on cariogenic organisms

Abstract

OBJECTIVE: to investigate the antibacterial effects of silver- and fluoride-containing compounds used in caries management. METHODS: Minimum inhibitory concentrations (MICs) of silver diammine fluoride (SDF), silver fluoride (AgF), silver nitrate (AgNO3), ammonium fluoride (NH4F) and sodium fluoride (NaF) were determined on Streptococcus mutans, Lactobacillus acidophilus and Actinomyces naeslundii. Serial (×2) dilutions from 2 400,000 µg•mL-1 (limits depending on solubility) were prepared in a 96-well plate with BHI media adjusted to pH 7 or pH 5 to simulate neutral and cariogenic conditions. Tests with silver compounds were duplicated in the dark to investigate the effect of light. RESULTS: Four-way analysis of variance (AoV) showed three main factors (agent, pH, species) to have highly-significant effects (p < 0.05) while light did not (p > 0.05). Dropping light and repeating as a three-way AoV, all factors remained significant as well as most interactions (p < 0.05). MIC(SDF) on the three bacteria ranged from 62 125 µg•mL-1, similar to AgNO3 (62 µg•mL-1) and AgF (31 62 µg•mL-1). For A. naeslundii and S. mutans, silver compounds had antibacterial effect at significantly lower concentration than the fluoride compounds (p < 0.001); NH4F (313 µg•mL-1) and NaF (500 µg•mL-1) had significantly weaker (p < 0.05) effects at pH 7 than at pH 5. Increased antibacterial effect was found for F at the lower pH (p < 0.001) but not for Ag. MICs for L. acidophilus could not be determined at pH 5 due to inadequate growth, while at pH 7 there was no growth for NaF (2 µg•mL-1) or NH4F (20 µg•mL-1), implying lower MIC values. CONCLUSIONS: Silver ions are antibacterial against oral microorganisms even at low concentration, while fluoride ion was only effective at relatively high concentration and affected by pH.