Effects of 630-,660-,810-,and 905-nm Laser Irradiation Delivering Radiant Exposure of 1-50 J/cm 2 on Three Species of Bacteria in Vitro

Abstract

Objective: To examine the effects of low-intensity laser therapy (LILT) on bacterial growth in vitro. Background Data: LILT is undergoing investigation as a treatment for accelerating healing of open wounds. The potential of coincident effects on wound bacteria has received little attention. Increased bacterial proliferation could further delay recovery; conversely inhibition could be beneficial.Materials and Methods: Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus were plated on agar and then irradiated with wavelengths of 630, 660, 810, and 905 nm (0.015 W/cm²) and radiant exposures of 1-50 J/cm². In addition, E. coli was irradiated with 810 nm at an irradiance of 0.03 W/cm² (1-50 J/cm²). Cells were counted after 20 h of incubation post LILT. Repeated measures ANOVA and Tukey adjusted post hoc tests were used for analysis. Results: There were interactions between wavelength and species (p = 0.0001) and between wavelength and radiant exposure (p = 0.007) in the overall effects on bacterial growth; therefore, individual wavelengths were analyzed. Over all types of bacteria, there were overall growth effects using 810- and 630-nm lasers, with species differences at 630 nm. Effects occurred at low radiant exposures (1-20 J/cm²). Overall effects were marginal using 660 nm and negative at 905 nm. Inhibition of P. aeruginosa followed irradiation using 810 nm at 5 J/cm² (-23%; p = 0.02). Irradiation using 630 nm at 1 J/cm² inhibited P. aeruginosa and E. coli (-27%). Irradiation using 810 nm(0.015 W/cm²) increased E. coli growth, but with increased irradiance (0.03 W/cm²) the growth was significant (p = 0.04), reaching 30% at 20 J/cm² (p = 0.01). S. aureus growth increased 27% following 905-nm irradiation at 50 J/cm². Conclusion: LILT applied to wounds, delivering commonly used wavelengths and radiant exposures in the range of 1-20 J/cm², could produce changes in bacterial growth of considerable importance for wound healing. A wavelength of 630 nm appeared to be most commonly associated with bacterial inhibition. The findings of this study might be useful as a basis for selecting LILT for infected wounds.