Interaction of the photobactericides methylene blue and toluidine blue with a fluorophore in Pseudomonas aeruginosa cells.

BACKGROUND AND OBJECTIVES: The difference in photobactericidal efficacy between methylene blue (MB) and toluidine blue (TB) may be explained by their involvement with proteins, lipopolysaccharides (LPS), and siderophores and siderophore-receptor protein complexes on the bacterial outer membrane. This study aims to determine if this is the case by using the fluorescence given off by a pseudomonal siderophore named pyoverdin. STUDY DESIGN/MATERIALS AND METHODS: Confocal laser scanning microscopy was used to observe the fluorescence of Pseudomonas aeruginosa cells excited at 488 nm in the presence of increasing dye concentrations. RESULTS: Cellular fluorescence at 522 nm progressively decreased with increasing dye concentrations. The Stern-Volmer constants for cellular fluorescence quenching with the dyes were compared to the association constants for dyes complexed with LPS. The quenching of cellular fluorescence was associated with the formation of a ground-state complex between the dyes and pyoverdin-FpvA protein system. MB readily complexed with this system, whereas TB complexed more strongly with LPS. CONCLUSION: The different affinities of the dyes for both pyoverdin-protein and LPS will affect the contributions of the dyes' interactions with these biopolymers to the overall bacterial photodamage.

Treatment of oral candidiasis with methylene blue-mediated photodynamic therapy in an immunodeficient murine model.

OBJECTIVES: The purpose of this study was to evaluate the efficacy of using methylene blue (MB)-mediated photodynamic therapy to treat oral candidiasis in an immunosuppressed murine model, mimicking what is found in human patients. STUDY DESIGN: Seventy-five experimental mice with severe combined immunodeficiency disease were inoculated orally with Candida albicans by swab 3 times a week for a 4-week period. On treatment day, mice were cultured for baseline fungal growth and received a topical oral cavity administration of 0.05 mL MB solution at one of the following concentrations: 250, 275, 300, 350, 400, 450, or 500 microgram/mL. After 10 minutes the mice were recultured and underwent light activation with 664 nm of diode laser light with a cylindrical diffuser. After photodynamic therapy the mice were cultured again for colony-forming units per milliliter and then killed, their tissue harvested for histopathology. RESULTS AND CONCLUSIONS: The results indicate an MB dose-dependent effect. Concentrations from 250 to 400 microgram/mL reduced fungal growth but did not eliminate Candida albicans. MB concentrations of 450 and 500 microgram/mL totally eradicated Candida albicans from the oral cavity, resulting in reductions from 2.5 log(10) and 2.74 log(10) to 0, respectively. These results suggest that MB-mediated photodynamic therapy can potentially be used to treat oral candidiasis in immunodeficient patients.

Comparison of the methylene blue and toluidine blue photobactericidal efficacy against gram-positive and gram-negative microorganisms.

BACKGROUND AND OBJECTIVE: Studies on the photobactericidal efficacy of methylene blue (MB) and toluidine blue (TB) have shown inconsistent results in the literature. This study evaluated the bactericidal efficacy of MB and TB against different bacteria under light and dark conditions to determine the most effective bactericidal dye. STUDY DESIGN/MATERIALS AND METHODS: Suspensions of Staphylococcus aureus, Streptococcus pneumoniae, Enterococcus faecalis, Hemophilus influenzae, Escherichia coli, and Pseudomonas aeruginosa in saline were treated in dark and red laser light conditions in the presence of each dye using an argon pumped-dye and a diode laser emitting light at 630 and 664 nm, respectively. The effect of dye concentration, dark incubation time, the fluence and intensity of laser light on the destruction of different bacteria were compared. RESULTS: Both dyes eradicated all examined bacteria under laser light. The complete photodestruction of microorganisms was reached at TB concentrations of 1.5-7-fold less than that of MB. CONCLUSION: TB exhibits a greater bactericidal activity than MB against most bacteria in dark and light conditions. Mostly, these results are consistent with their respective dye partition coefficients.