In Vitro Inhibitory Effect of Silver Diamine Fluoride Combined with Potassium Iodide against Mixed-Species Biofilm Formation on Human Root Dentin

Abstract

Applying a saturated potassium iodide (KI) solution immediately after silver diamine fluoride (SDF) application may affect the inhibitory effects of SDF on biofilm formation. This study compared the efficacy of 38% SDF with and without KI on preventing mixed-species biofilm formation on human root dentin surfaces and assessed ion incorporation into root dentin. The biofilms, composed of Streptococcus mutans, Lactobacillus rhamnosus, and Actinomyces naeslundii, were grown on specimen surfaces treated with either SDF or SDF + KI. After 24 h, the biofilms were evaluated using scanning electron microscopy, live/dead staining, adenosine triphosphate (ATP) assays, colony-forming unit (CFU) counts, and quantitative polymerase chain reaction. A Mann-Whitney U test was used to compare the results between the groups. Ion incorporation was assessed using an electron probe microanalyzer. The relative ATP content in the SDF + KI group was significantly higher than that in the SDF group (p < 0.05). However, biofilm morphology and the logarithmic reduction in CFUs and bacterial DNA were comparable across the groups. The SDF + KI treatment resulted in less silver and fluoride ion incorporation than that yielded by SDF alone. The inhibitory effects of SDF and SDF + KI on mixed-species biofilm formation were almost equivalent, although KI application affected the ion incorporation.

Keywords: biofilm; potassium iodide; root caries; silver diamine fluoride.

Figure 1

Representative scanning electron microscopy (SEM) images of cariogenic biofilms formed on root dentin surfaces in the SDF (c,d), SDF + KI (g,h), and corresponding control groups (a,b,e,f) after 24 h of incubation. White arrows indicate the biofilm clusters. SDF: silver diamine fluoride; KI: potassium iodide; scale bars = 100 µm.

Figure 2

Confocal laser scanning microscopy (CLSM) analysis of mixed-species biofilms consisting of S. mutans, L. rhamnosus, and A. naeslundii formed on root dentin surfaces. (a–p) Representative three-dimensional reconstructed images corresponding to live/dead staining; scale bars = 20 µm (top view) and 30 µm (3D view). The green signal is due to the SYTO9 dye which indicates live cells, while the red signal is due to propidium iodide which marks the dead cells. (q) Ratio of dead to live cells. SDF: silver diamine fluoride; KI: potassium iodide.

Figure 3

Relative ATP content of biofilm on root dentin surfaces. Data from the control of each test group were used as the standard for calculating the relative content in comparison with the other groups. ATP: adenosine triphosphate; SDF: silver diamine fluoride; KI: potassium iodide.

Figure 4

Log reduction in viable cell number (CFU/mL) of S. mutans, L. rhamnosus, and A. naeslundii in the SDF and SDF + KI groups (n = 5). Medians, quartiles, and extreme values are given. SDF: silver diamine fluoride; KI: potassium iodide.

Figure 5

Log reduction in DNA concentration (copies/mL) of S. mutans, L. rhamnosus, A. naeslundii, and 16S rDNA in the SDF and SDF + KI groups (n = 5). Medians, quartiles, and extreme values are given. SDF: silver diamine fluoride; KI: potassium iodide.

Figure 6

Silver and fluoride distribution profiles in the longitudinal section of root dentin specimens after incubation for 24 h in the control (a–d), SDF (b–e), and SDF + KI (c–f) groups. Arrowheads indicate the disc surface. SDF: silver diamine fluoride; KI: potassium iodide.

Figure 7

Experimental flow chart. MRD: modified Robbins device; SDF: silver diamine fluoride; KI: potassium iodide; Sm: S. mutans; Lr: L. rhamnosus; An: A. naeslundii; BHI: brain–heart infusion; SEM: scanning electron microscopy; CLSM: confocal laser scanning microscopy; ATP: adenosine triphosphate; qPCR: quantitative polymerase chain reaction; EPMA: electron probe microanalyzer.