Thiocyanate potentiates antimicrobial photodynamic therapy: In situ generation of the sulfur trioxide radical anion by singlet oxygen

Antimicrobial photodynamic therapy (PDT) is used for the eradication of pathogenic microbial cells and involves the light excitation of dyes in the presence of O₂, yielding reactive oxygen species including the hydroxyl radical (OH) and singlet oxygen (¹O₂). In order to chemically enhance PDT by the formation of longer-lived radical species, we asked whether thiocyanate (SCN⁻) could potentiate the methylene blue (MB) and light-mediated killing of the gram-positive Staphylococcus aureus and the gram-negative Escherichia coli. SCN⁻ enhanced PDT (10µM MB, 5J/cm² 660nm hv) killing in a concentration-dependent manner of S. aureus by 2.5log₁₀ to a maximum of 4.2log₁₀ at 10mM (P