Evaluation of the Inactivation of Microorganisms by a Blue Laser (445 nm)—An In Vitro Study

<b>Background</b>: Blue laser light has been the subject of research regarding the inactivation of microorganisms as a possible alternative to chemical treatment methods for a number of years. In dentistry, blue light could be used, for example, in the treatment of periodontitis/peri-implantitis, as well as in endodontics and against caries. It could serve as an alternative or supplement to traditional chemical and/or invasive methods. The antimicrobial effectiveness of a blue laser in relation to the speed of treatment is investigated using three different microbial test organisms in order to identify possible species differences. <b>Methods</b>: The test organisms <i>Enterococcus faecalis</i>, <i>Streptococcus mutans</i>, and <i>Candida albicans</i> were applied to smooth zirconium discs and treated twice with a diode laser at 445 nm wavelength with a traversing speed of 1, 2, and 4 mm/s. The antimicrobial effect was analysed based on the resulting colony-forming units on agar plates. The temperature was measured during the treatment. Preliminary tests were carried out using the MTT dye test to determine relevant setting parameters and the required energy dose. <b>Results</b>: Statistically significant differences were found between the negative control and the treated samples for all three tested organisms, with a maximum viability reduction of 1.8 log₁₀ CFU/mL for <i>Enterococcus faecalis,</i> 2.5 log₁₀ CFU/mL for <i>Streptococcus mutans</i>, and 1.0 log₁₀ CFU/mL for <i>Candida albicans</i> at 1 mm/s traversing speed, regarding estimated marginal means (<i>p</i> < 0.001). The temperature on the substrate surface reached 30 to 42 °C for all samples evaluated. <b>Conclusions</b>: Blue laser light (445 nm) demonstrates antimicrobial activity, which increases with prolonged exposure. Further research is needed to assess all key influencing parameters and define possible clinical applications.