Effects of the XeCl excimer laser on Streptococcus mutans

The effect of XeCl excimer laser irradiation on the growth of Streptococcus mutans in liquid media and on agar plates was studied. Bacterial suspensions of S. mutans were placed in 96 wells of well culture plates. The contents of 72 wells (three experimental groups of 24 wells each) were lased for different time durations (2, 4, and 8 s). The remaining 24 wells were left unlased to be used as controls. Samples were withdrawn from all wells and examined for surviving bacteria. In addition, blood agar plates were inoculated with S. mutans and were lased with different energy densities (fluences). Zones of bacterial inhibition were measured. Analysis of variance test was used to determine statistical differences. The bactericidal effect of the laser applications was directly related to the amount of radiation time. Laser irradiation for 4 and 8 s resulted in bactericidal effect that was statistically significant compared with no treatment or to 2-s exposure. The effect of different energy levels was studied by irradiating inoculated blood agar plates. The zones of inhibition produced by higher energy levels (0.5 J/cm2, 0.7 J/cm2, and 1.0 J/cm2) were larger in comparison to the lowest fluence used (0.1 J/cm2). Application of the laser to the surface of the agar plates produced an indentation with a surrounding halo. The indentations and the zones of inhibition were more pronounced as the fluences increased. Based on our results it appears that the XeCl 308-nm excimer laser can kill S. mutans. This effect should be tested on other bacteria commonly present infected root canals.