Comparative cell targeting in vitro using the CO2 laser

In keeping with current interest in bactericidal effects of lasers to treat localized infection, we examined the effect of CO2 laser light on three bacterial species. Cell monolayers were placed atop filters minimizing the presence of extracellular water. Samples were exposed to irradiance values between 0 and 520 W/cm2 for 0.1 s. Cell survival was determined by colony counts. There was logarithmic killing of the bacteria with increasing irradiance; 90% kill was effected at 180 W/cm2 for Escherichia coli, 350 W/cm2 for Streptococcus mutans, and 250 W/cm2 for Bacillus stearothermophilus. However, standard deviations suggest no significant difference between cell types. Our results show the bactericidal effect of the CO2 laser with no remarkable difference between Gram types. However, substantial cell kill was achieved at relatively low irradiance. Whereas selectivity of damage may not be afforded by the CO2 laser, it may minimize collateral damage.