Visible light therapy (400-700 nm), i.e. photodynamic therapy, phototherapy, etc. has been used experimentally and clinically for treatment in acne, cancer, wounds, jaundiced neonates and other ailments. In the wake of increasing antibiotic resistance, the application of blue light (400-500 nm) as an antimicrobial strategy is appealing. Previous studies have elucidated differences on the responses of bacteria to irradiation with blue light (or blue light irradiation-BLI), ranging from a decrease in growth for some species, to stimulating proliferation in others. Although effective against a range of Gram-positive pathogens, BLI appears to be less effective at targeting Gram-negative bacteria and the basis of this phenomenon remains unknown.
Studies evaluating the phototoxic effect of BLI on bacteria revealed that endogenous photosensitizers absorbing blue light lead to the formation of reactive oxygen species (ROS). In turn, these ROS, specifically singlet oxygen, can have a cytotoxic effect. Conversely, significant literature documents the ability of bacteria to sense and respond to blue light, via the use of BLUF (sensor of blue light using FAD-flavin adenine dinucleotide) domain-containing proteins. Much of the work that has been done has focused on bacteria early in its life cycle, but it is bacteria in later stages of growth that are responsible for causing infections and diseases.
Through this work the growth phase dependencies on reductions in Gram-negative Escherichia coli were defined. The reduction differences between non-pathogenic and pathogenic E. coli strains were determined. The endogenous photosensitizer that is involved in the BLI-induced response at the 455 nm wavelength was also identified. Light parameters such as wavelength, energy dose, and the energy flux affect reductions in growth; findings from modulating these parameters were used to further enhance reductions in E. coli post-BLI. Through these studies and additional kinetic analysis, a proposed a model has been developed to describe the observed BLI reductions.