Mechanism of high-power NIR laser bacteria inactivation

Lasers are used in dentistry for a variety of indications. One of these is the disinfection of root canals or the sterilization of residual caries. Many studies have demonstrated the capacity to kill bacteria for lasers but the fundamental mechanism of the laser effect remains quite unclear. With our experiments we wanted to determine whether high-power NIR laser bacterial killing is caused by the light itself (photochemical effect) or by a photothermal process. In order to differentiate between mechanisms we heated bacteria suspensions of a nonpathogenic strain of E. coli by a water bath and by a diode laser (940 nm) with the same temporal temperature course. Furthermore, bacteria suspensions were irradiated while the temperature was fixed by ice water. Killing of bacteria was measured via fluorescence labelling. Comparison of killing rates between laser and water-based heating shows no significant differences. The most important parameter is the maximum temperature. Laser irradiation of bacteria at low temperatures does not result in killing. Our experiments show that at least for E. coli bacteria inactivation by high-power laser irradiation is solely based on a thermal process.