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 hydroxy apatite 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 min. 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% to 2% for both 1 and 3 min killed significantly more bacteria when compared to the controls (P < 0.05). Cell viability tended to be lower after the application of NaOCl for 3 min than that for 1 min. Conclusion Our experiments using an ex vivo model showed that within the range of 0.1% to 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.