The effect of hydrogen peroxide/colloidal silver on reducing the colonization and growth of heterotrophic bacteria in dental unit waterlines

The objective of the present study was to investigate reduction in the colonization and growth of heterotrophic bacteria in dental unit waterlines by using hydrogen peroxide/colloidal silver as disinfectant. Twenty-seven dental units were included; 6 units that were more than 20 years old and 7 units that had been in use for 2 years comprised the old and new treatment groups, respectively. Fourteen units served as controls. The treatment groups were disinfected continuously and every 4 weeks shock doses were applied over a 20-week period. Water samples were taken before treatment, 1 and 2 weeks after treatment, and thereafter every 4 weeks; then they were inoculated onto R2A agar plates. While 1–16-week results for the old treatment group showed total heterotrophic bacterial counts of higher than 1 × 10^5 cfu/mL, at 20 weeks they were below 7.5 × 102 cfu/mL. Only 2 units were able to reach levels of ≤200 cfu/ mL, which is the dental unit water quality standard. For the new treatment group it was achieved for all units after 1 week. Electron microscopic analysis also revealed that while biofilm formation was more evident in the old treatment group, after a longer treatment period biofilms were eliminated completely. The findings indicate that disinfection was effective in improving the output water quality using hydrogen peroxide/colloidal silver disinfectant.