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. | Turkish Journal of Biology Turk J Biol (2013) 37: 336-341 © TÜBİTAK doi: Research Article The effect of hydrogen peroxide/colloidal silver on reducing the colonization and growth of heterotrophic bacteria in dental unit waterlines* 1, 2 3 1 Meral ÖZALP **, Ömer Engin BULUT , Atilla Stephan ATAÇ , Melike EKİZOĞLU , 1 4 4 Didem KART , Hakan Hamdi ÇELİK , İlkan TATAR 1 Department of Pharmaceutical Microbiology, Faculty of Pharmacy, Hacettepe University, 06100, Sıhhiye, Ankara, Turkey 2 Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Başkent University, 06490, Bahçelievler, Ankara, Turkey 3 Department of Pediatric Dentistry, Faculty of Dentistry, Hacettepe University, 06100, Sıhhiye, Ankara, Turkey 4 Department of Anatomy, Faculty of Medicine, Hacettepe University, 06100, Sıhhiye, Ankara, Turkey Received: Accepted: Published Online: Printed: Abstract: 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 × 105 cfu/mL, at 20 weeks they were below × 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 .