Photomedicine and Laser Surgery Volume 25, Number 6, 2007 © Mary Ann Liebert, Inc. Pp. 513–518 DOI: 10.1089/pho.2007.2109 The Impact of Photodynamic Therapy on the Viability of Streptococcus mutans in a Planktonic Culture I.M. BEVILACQUA, D.D.S., M.Sc., 1,2 R.A. NICOLAU, D.D.S., Ph.D., 1,2 S. KHOURI, M.Sc., 1 A. BRUGNERA JR., 2 G.R. TEODORO, 1 R.A. ZÂNGARO, Ph.D. 1,2 and M.T.T. PACHECO, Ph.D. 2 ABSTRACT Objective: This study investigated the effect of photodynamic therapy (PDT) with toluidine blue O (TBO) and a light-emitting diode (LED) on the viability of Streptococcus mutans cells in a planktonic culture. Background Data: Growth of Streptococcus mutans is the first step in the development of tooth decay. The use of light and dyes promotes cellular death in a noninvasive way, reducing treatment time. Methods: The LED used in this study had output power of 116 mW, its energy was 21 J, and the fluency was 2.18 J/cm 2 . Samples were pre- pared and divided into five groups: (1) control group (); (2) control group (); (3) TBO; (4) LED; and (5) LED  TBO. Results: One hundred percent of the bacteria were killed following irradiation with LED and TBO. The biofilm that formed on the glass surfaces was analyzed by SEM and colony count. Conclusions: It was demonstrated that PDT was efficient at killing microorganisms and preventing the formation of biofilms. 513 INTRODUCTION M ANY MICROBIAL SPECIES are present in the biofilm that forms on teeth. The effectiveness of antimicrobial therapy, both topical and systemic, tends to be minimized by this biofilm, and has led to the evaluation of new therapeutic modalities. 1 Dental plaque formation is one of the initial phases of tooth decay, which is a microbial disease that affects a tooth’s calci- fied tissues. Streptococcus mutans is one of the most important bacteria present in dental plaque, and its growth is the first step in the development of tooth decay. It causes demineralization of the inorganic part of the tooth, and if untreated it progresses to destruction of the organic portion. 2 Thus, elimination of path- ogenic microorganisms on teeth is fundamental to prevention and control of tooth decay. 3 There are limitations to removing dental plaque using man- ual instruments. For this reason, use of lasers or LEDs of dif- ferent wavelengths, in association with various photosensi- tizing dyes, can play an important role as an alternative treatment. 4–7 The dyes work as optical absorption agents, and are activated by irradiation with light of a specific wave- length, resulting in the generation of cytotoxic species, in- cluding singlet oxygen and free radicals, that exert a bacte- ricidal effect, but that are not toxic to host cells. LEDs are an alternative source of light in PDT used to reduce the num- ber of bacteria. 9 The use of the LED’s nonpolarized light has proven to be effective therapy, since to produce an antibac- terial effect, the light source must have a wavelength in the absorption band of the dye being used. 9,10 Thus, the purpose of this study was to evaluate the effect in vitro of LED light (640  20 nm) in association with toluidine blue O (TBO) on the viability of Streptococcus mutans (S. mutans) cells in a planktonic culture. MATERIALS AND METHODS Construction of samples The samples were made of glass capillary tubes with 1.0 mm internal diameter, 1.5 mm external diameter, and approximately 2.3 cm long that were curved on one end (0.5 cm). All samples were sterilized in a vertical autoclave (Phoenix AV 75), and then put into test tubes. Photosensitising dye The dye used in this study was toluidine blue O (TBO-Fór- mula & Ação) at a concentration of 100 g/mL. 1,2,11,12 1 Health and Sciences College, and 2 Dental Laser Center, Institute of Research and Development, Vale do Paraíba University (UNIVAP), Ur- banova, São José dos Campos, Brazil.