Paradoxical potentiation of methylene blue-mediated antimicrobial photodynamic inactivation by sodium azide: Role of ambient oxygen and azide radicals

Abstract

Sodium azide (NaN3) is widely employed to quench singlet oxygen during photodynamic therapy (PDT), especially when PDT is used to kill bacteria in suspension. We observed that addition of NaN3 (100 μM or 10 mM) to gram-positive Staphylococcus aureus and gram-negative Escherichia coli incubated with methylene blue (MB) and illuminated with red light gave significantly increased bacterial killing (1–3 logs), rather than the expected protection from killing. A different antibacterial photosensitizer, the conjugate between polyethylenimine and chlorin(e6) (PEI-ce6), showed reduced PDT killing (1–2 logs) after addition of 10 mM NaN3. Azide (0.5 mM) potentiated bacterial killing by Fenton reagent (hydrogen peroxide and ferrous sulfate) by up to 3 logs, but protected against killing mediated by sodium hypochlorite and hydrogen peroxide (considered to be a chemical source of singlet oxygen). The intermediacy of N3• was confirmed by spin-trapping and electron spin resonance studies in both MB-photosensitized reactions and Fenton reagent with addition of NaN3. We found that N3• was formed and bacteria were killed even in the absence of oxygen, suggesting the direct one-electron oxidation of azide anion by photoexcited MB. This observation suggests a possible mechanism to carry out oxygen-independent PDT.

Potassium Iodide Potentiates Antimicrobial Photodynamic Inactivation Mediated by Rose Bengal in In Vitro and In Vivo Studies

“…We had previously shown that the potentiation of aPDI mediated by MB was paradoxically potentiated by 10 mM sodium azide (singlet oxygen quencher) operating by a one-electron transfer from azide anion to excited-state MB to form azide radicals in an oxygen-independent process (31). Therefore, we assumed that the mechanism in the case of KI and MB was an analogous one-electron transfer from the iodide anion to the excited-state PS to form an iodine radical and an MB radical anion (16).…”

Honey-sensitive Pseudomonas aeruginosa mutants are impaired in catalase A

“…Mannitol and sodium azide were chosen as it has been demonstrated that they could efficiently protect bacterial cells from oxidative stress ascribable to superoxide anion and hydroxyl radicals and to singlet oxygen, respectively (Huang et al, 2012;Tavares et al, 2011;Ishikawa et al, 2010). Cells were grown in the presence of honey and of concentrations of mannitol that did not affect the growth or of sublethal concentrations of sodium azide.…”

Broad-Spectrum Antimicrobial Effects of Photocatalysis Using Titanium Dioxide Nanoparticles Are Strongly Potentiated by Addition of Potassium Iodide

“…In order to gain some information on whether the addition of iodide increased the formation of reactive oxygen species (ROS) produced by the illuminated TiO 2 (for instance by acting as an electron donor) or, conversely, whether the ROS produced by the photocatalysis was responsible for oxidizing the iodide to give iodine radicals, iodine, and hypoiodite, we used two fluorescent probes for ROS that we had previously used in photodynamic therapy studies (21,22) and asked whether their activation would be quenched by addition of iodide. SOSG is relatively specific for singlet oxygen, and HPF is relatively specific for detecting hydroxyl radicals (23).…”