title: Paradoxical potentiation of methylene blue-mediated antimicrobial photodynamic inactivation by sodium azide: role of ambient oxygen and azide radicals.
abstract: Sodium azide (NaN(3)) 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 NaN(3) (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 10mM NaN(3). Azide (0.5mM) 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 N(3)() was confirmed by spin-trapping and electron spin resonance studies in both MB-photosensitized reactions and Fenton reagent with addition of NaN(3). We found that N(3)() 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.
authors.affiliation: Department of Infectious Diseases, First Affiliated College & Hospital, Guangxi Medical University, Nanning, China 530021.
authors.forename: Liyi
authors.initials: L
authors.lastname: Huang
authors.forename: Tyler G
authors.initials: TG
authors.lastname: St Denis
authors.forename: Yi
authors.initials: Y
authors.lastname: Xuan
authors.forename: Ying-Ying
authors.initials: YY
authors.lastname: Huang
authors.forename: Masamitsu
authors.initials: M
authors.lastname: Tanaka
authors.forename: Andrzej
authors.initials: A
authors.lastname: Zadlo
authors.forename: Tadeusz
authors.initials: T
authors.lastname: Sarna
authors.forename: Michael R
authors.initials: MR
authors.lastname: Hamblin
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry / Q000494:pharmacology* / Q000528:radiation effects; D004357:Drug Synergism; D004926:Escherichia coli / Q000187:drug effects; D005609:Free Radicals / Q000737:chemistry / Q000494:pharmacology; D006861:Hydrogen Peroxide / Q000494:pharmacology; D007501:Iron / Q000494:pharmacology; D008751:Methylene Blue / Q000737:chemistry / Q000494:pharmacology* / Q000528:radiation effects; D050296:Microbial Viability / Q000187:drug effects; D010084:Oxidation-Reduction; D010778:Photochemotherapy; D017319:Photosensitizing Agents / Q000494:pharmacology*; D017382:Reactive Oxygen Species / Q000737:chemistry / Q000378:metabolism*; D019810:Sodium Azide / Q000494:pharmacology*; D012973:Sodium Hypochlorite / Q000494:pharmacology; D013211:Staphylococcus aureus / Q000187:drug effects
chemical_list: D000900:Anti-Bacterial Agents; C045076:Fenton's reagent; D005609:Free Radicals; D017319:Photosensitizing Agents; D017382:Reactive Oxygen Species; D019810:Sodium Azide; D006861:Hydrogen Peroxide; D012973:Sodium Hypochlorite; D007501:Iron; D008751:Methylene Blue