Effects of Sodium Hypochlorite Concentration and Application Time on Bacteria in an Ex Vivo Polymicrobial Biofilm Model.

INTRODUCTION: In endodontic treatment, it is important to remove or inactivate biofilms in the root canal system. We investigated the effects of different concentrations and application times of sodium hypochlorite (NaOCl) on the viability of bacteria in ex vivo polymicrobial biofilms of different maturation levels. METHODS: Polymicrobial biofilms were prepared from dental plaque samples and grown for 1, 2, and 3 weeks under anaerobic conditions on collagen-coated hydroxyapatite discs as an ex vivo biofilm model. The biofilms were then exposed to NaOCl at concentrations ranging from 0.1% to 2% for 1 or 3 minutes. The control group was exposed to sterile distilled water. Viability staining was performed and examined by confocal laser scanning microscopy to determine the percentage of biofilm bacteria killed by NaOCl. Scanning electron microscopy was also performed to visually examine the biofilms. RESULTS: Application of NaOCl at 0.5%-2% for both 1 and 3 min killed significantly more bacteria when compared to the controls (P < .05). Cell viability tended to be lower after the application of NaOCl for 3 minutes than that for 1 minute. CONCLUSIONS: Our experiments using an ex vivo model showed that within the range of 0.1%-2% of NaOCl, higher NaOCl concentrations and longer application times were more effective in killing biofilm bacteria, and that mature biofilms were more resistant to NaOCl than younger biofilms.

Morphological Analyses of Effects of Endodontic Irrigant Solutions Using a Root Canal Model and an Immersion Model.

Objective: This study aimed to compare an experimental model simulating clinical root canal irrigation (root canal model) with a conventional experimental model immersing dentin sample to irrigants (immersion model) to evaluate removal of the smear layer and decalcification of the root canal dentin using sodium hypochlorite (NaOCl) and two different concentrations of ethylenediaminetetraacetic acid (EDTA) solution. Materials and Methods: Forty-five single-rooted extracted human teeth were prepared using a Ni-Ti rotary file. EDTA, NaOCl, and citric acid were used in the root canal models and the immersion models. After the irrigation protocol, root canal surfaces were observed under scanning electron microscopy. Residual smear and decalcification of the root canal dentin were evaluated objectively by measuring the percentage of the area occupied by visible dentin tubules, the number of visible dentin tubules, and the mean area of a visible single dentin tubule. Results: Root canal and immersion models with the same irrigation protocol showed significantly different results for smear residues and decalcification of root canal dentin. In the root canal model, neither different EDTA concentrations nor the order of EDTA and NaOCl applications significantly impacted smear residues or decalcification of root canal dentin. Furthermore, no erosion of the root canal dentin surface was observed in any experimental groups in the root canal model using EDTA and NaOCl compared to intact dentin. Conclusions: Experimental design affected results for residual smear layer and decalcification of root canal dentin. The order of EDTA and NaOCl use and the concentration of EDTA did not affect results. EDTA and NaOCl irrigation did not cause erosion in the root canal model in this study.

An extensive description of the microbiological effects of silver diamine fluoride on dental biofilms using an oral in situ model.

Silver diamine fluoride (SDF) has been long studied in laboratories, and its clinical effectiveness in the treatment and prevention of root caries has been reported. In the present study, we assessed the microbiological effects of SDF on dental biofilms grown on demineralized dentin in situ. Specifically, demineralized bovine root dentin slabs used as biofilm substrates were treated with 38% SDF, and the biofilms formed after this treatment were analyzed via real-time PCR, DEAD/LIVE cell staining, and SEM. Next, the viable cell count was determined, and microbial profiles were compared using 16S rRNA gene sequencing. Untreated slabs were used as controls. We observed significant decreases in viable cell counts (p < 0.05), number of biofilm-forming cells (p < 0.01), biofilm thickness (p < 0.01), and high proportion of dead cells with SDF treatment (p < 0.01). The microcolonies in the SDF-treated biofilms showed less complexity, and only a limited number of genera were differentially abundant between the groups. Microbial diversity index comparisons showed no significant differences between the groups with respect to treatments days (p = 0.362). Thus, SDF negatively influenced dental biofilm growth on demineralized root dentin in situ; however, its antimicrobial action did not target a specific oral taxon.