Mechanism of transient photothermal inactivation of bacteria using a wavelength-tunable nanosecond pulsed laser

The Biofilm Removal and Bactericidal Effect of an 810-nm High-Power Laser on an Orthodontic Bracket Surface: An In Vitro Study

Objective: The present study aimed to analyze the biofilm removal and bactericidal effect of laser treatment alone and laser combined with ultrasonic scaling on orthodontic brackets. It also assessed whether the use of a laser can improve the efficiency of biofilm removal and bactericidal effect compared with traditional ultrasonic instrumentation. Background Data: Streptococcus mutans (S. mutans) can lead to white spots and dental caries. Orthodontic brackets make teeth cleaning more difficult, and biofilms or bacteria on the surface of brackets worsen the oral environment, which may cause some oral diseases. Laser can be used for biofilm removal and killing bacteria on the surface of an object through thermal, photochemical, and pressure effects, which is widely used in the treatment of oral diseases. Methods: A total of 600 mandibular incisor brackets were collected for this study. Among these, 320 unused brackets were used for the S. mutans crystal violet assay (n = 160) and for S. mutans live/dead bacterial staining (n = 160). Another 280 brackets, obtained from patients who had undergone therapy for over two years, were used for the mature multispecies biofilms removal assay (n = 120) and multispecies bacterial live/dead bacterial staining (n = 160). Ultrasonic scaling, laser, and laser combined with ultrasonic scaling were applied to the labial surface of brackets covered by S. mutans biofilm or mature multispecies biofilms. Specifically, we used the following three methods: ultrasonic scaling for 10 sec without laser; 810-nm laser (Doctor Smile, Italy, LA5D0 001.1) with 0.3-mm spot size at total 21.2 kJ/cm2 for 10 sec; and 810-nm laser at total 10.6 kJ/cm2 for 5 sec, followed by ultrasonic scaling for 5 sec. The 810-nm diode laser removed biofilms with a power of 1.5 W and a power density of 2.12 kW/cm2. The S. mutans biofilm was examined using crystal violet assay, and scanning electron microscopy (SEM) was used for mature multispecies biofilms to evaluate the effect of the three methods on biofilm removal. Live/dead bacterial staining was used to examine the bactericidal effect on remaining biofilms by confocal laser scanning microscopy (CLSM). Results: For S. mutans biofilm, the optical density (OD) value and live/dead bacterial ratio in the laser and the laser combined with ultrasonic scaling groups were significantly lower than those in the ultrasonic scaling group (p < 0.05); moreover, the OD value and the live/dead bacterial ratio in laser treatment combined with ultrasonic scaling and laser treatment alone showed no significant difference (p > 0.05). For mature multispecies biofilms, the percentage of biofilm coverage after treatment was higher in the laser group than in the ultrasonic scaling group (p < 0.05) and lower in the laser combined with ultrasonic scaling group than in the ultrasonic scaling group (p < 0.05), and live/dead bacterial staining showed that laser treatment alone killed the most bacteria, followed by laser treatment combined with ultrasonic scaling, while ultrasonic scaling alone seldom killed bacteria. Conclusions: Laser treatment alone has a better bactericidal effect and can also remove more S. mutans biofilm than ultrasonic scaling alone, but it fails to remove more mature multispecies biofilms. Laser treatment combined with ultrasonic scaling can remove more S. mutans biofilm and mature multispecies biofilms than ultrasonic scaling alone and also has a better bactericidal effect than ultrasonic scaling alone on a bracket surface.

Time-dose reciprocity mechanism for the inactivation of Escherichia coli explained by a stochastic process with two inactivation effects

There is a great demand for developing and demonstrating novel disinfection technologies for protection against various pathogenic viruses and bacteria. In this context, ultraviolet (UV) irradiation offers an effective and convenient method for the inactivation of pathogenic microorganisms. The quantitative evaluation of the efficacy of UV sterilization relies on the simple time-dose reciprocity law proposed by Bunsen-Roscoe. However, the inactivation rate constants reported in the literature vary widely, even at the same dose and wavelength of irradiation. Thus, it is likely that the physical mechanism of UV inactivation cannot be described by the simple time-dose reciprocity law but requires a secondary inactivation process, which must be identified to clarify the scientific basis. In this paper, we conducted a UV inactivation experiment with Escherichia coli at the same dose but with different irradiances and irradiation durations, varying the irradiance by two to three orders of magnitude. We showed that the efficacy of inactivation obtained by UV-light emitting diode irradiation differs significantly by one order of magnitude at the same dose but different irradiances at a fixed wavelength. To explain this, we constructed a stochastic model introducing a second inactivation rate, such as that due to reactive oxygen species (ROS) that contribute to DNA and/or protein damage, together with the fluence-based UV inactivation rate. By solving the differential equations based on this model, the efficacy of inactivation as a function of the irradiance and irradiation duration under the same UV dose conditions was clearly elucidated. The proposed model clearly shows that at least two inactivation rates are involved in UV inactivation, where the generally used UV inactivation rate does not depend on the irradiance, but the inactivation rate due to ROS does depend on the irradiance. We conclude that the UV inactivation results obtained to date were simply fitted by one inactivation rate that superimposed these two inactivation rates. The effectiveness of long-term UV irradiation at a low irradiance but the same dose provides useful information for future disinfection technologies such as the disinfection of large spaces, for example, hospital rooms using UV light, because it can reduce the radiation dose and its risk to the human body.

Action Spectra of Bacteria and Purification of Pollutant Water at Faucets Using a Water Waveguide Method

Ultraviolet (UV) radiation treatment is an effective method for purifying pollutant water contaminated with bacteria and/or chemicals. As an emerging technology, purification by deep ultraviolet light-emitting diodes (DUV-LEDs) is promising. Few studies have used the point-source characteristics of LEDs and have instead replaced mercury vapor lamps with LEDs. Here, we show our recent progress in the instantaneous purification of contaminated water by combining the point-source characteristics of DUV-LEDs with a water waveguide (WW). Before the demonstration, we determined the efficacy of disinfection as a function of irradiation wavelength (action spectra) by constructing a wavelength tunable DUV light source. We found that, as a function of irradiation wavelength, there is a strong correlation between the dose-based inactivation rate constants and deoxyribonucleic acid (DNA) absorbance. Based on this correlation, the emission wavelength of 265 nm was determined as the most effective wavelength for disinfecting water contaminated with bacteria. Instantaneous 2-log disinfection levels of water contaminated with Escherichia coli O1 or Pseudomonas aeruginosa were demonstrated by using the DUV-LED WW method. We also discuss how far-UVC radiation shorter than 230 nm, which has recently been attracting attention and is known as a safe and effective disinfection wavelength for the human body, cannot give a higher-dose-based inactivation rate constant compared to that of 265 nm irradiation due to the larger absorption coefficient of water with a wavelength shorter than 230 nm.