Sensitivity of oral bacteria to 254 nm ultraviolet light

Aim  To explore the sensitivity of bacteria commonly found in root canals to 254 nm ultraviolet (UV) light, either as individual cells or as participants of a bacterial multilayer. Methodology  The sensitivity of oral bacteria, as individual cells, to UV light was tested by subjecting plates streaked with bacteria to 254 nm UV, at a fluence of 1–20 mJ cm −2 . An experimental model was designed to produce a bacterial multilayer and to study absorption of UV light by bacteria in an outer layer and its effect on the elimination of bacteria in the inner layer. Results  Direct exposure to relatively low doses of UV light (2–7 mJ cm −2 ) effectively eliminated all bacterial strains tested. Furthermore, an Enterococcus faecalis strain, partially resistant to a 24 h exposure to calcium hydroxide, was effectively eliminated within several seconds of exposure to UV light ( P  < 0.001). UV was absorbed by a multilayer of bacteria. When 4 bacterial cells  μ m −2 were present in the light path, the UV light dose had to be increased by a factor of ×10 to achieve 100% elimination of the bacteria in an inner layer. Conclusions  The application of UV light to eliminate endodontic pathogens may be possible. Nevertheless, its absorbance by outer layers of bacteria should be considered and the UV light dose adapted accordingly.