Dye-mediated bactericidal effect of He-Ne laser irradiation on oral microorganisms

Little attention has been given to the bactericidal effect of laser irradiation, particularly using low-power energy lasers. It has been demonstrated that He-Ne laser light has an inhibitory action on dental plaque. The purpose of this study was to investigate the bactericidal effect of He-Ne laser irradiation on cariogenic microorganisms. The bactericidal effect was determined by the formation of a growth-inhibitory zone or by the counting of viable bacterial colonies. Streptococcus sobrinus AHT that is a Gram-positive microorganism was sensitive to He-Ne laser light, but Escherichia coli, a Gram-negative microorganism, was resistant. The effect of several dyes necessary to instigate a bactericidal action was also examined. A growth-inhibitory zone was observed using 10 kinds of blue, purple, or green dyes, which were mainly phenylmethane dyes. The leakage of potassium from S. sobrinus AHT following laser irradiation was determined using an atomic absorption spectrophotometer. The leakage began to increase following irradiation for 2 min, and reached a plateau following irradiation for 30-60 min. Moreover, to examine some changes in the dye itself following laser irradiation in the absence of bacteria, ultraviolet-visible absorption spectra and 1H NMR spectra were recorded. In this study, it was indicated that the bactericidal effect on cariogenic bacteria by He-Ne laser irradiation was efficient only in the presence of specific dyes. It is suggested that this laser may be suitable for clinical applications in preventive dentistry.