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 O-2, yielding reactive oxygen species including the hydroxyl radical ((OH)-O-center dot) and singlet oxygen (O-1(2)). 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 (10 mu M MB, 5J/cm(2) 660 nm by) killing in a concentration-dependent manner of S. aureus by 2.5 log(10) to a maximum of 4.2 log(10) at 10 mM (P<0.001) and increased killing of E. coli by 3.6 log(10) to a maximum of 5.0 log(10) at 10 mM (P<0.01). We determined that sCN(-) rapidly depleted O-2 from an irradiated MB system, reacting exclusively with O-1(2), without quenching the MB excited triplet state. SCN- reacted with O-1(2), producing a sulfur trioxide radical anion (a sulfur-centered radical demonstrated by EPR spin trapping). We found that MB-PDT of SCN- in solution produced both sulfite and cyanide anions, and that addition of each of these salts separately enhanced MB-PDT killing of bacteria. We were unable to detect EPR signals of (OH)-O-center dot, which, together with kinetic data, strongly suggests that MB, known to produce (OH)-O-center dot and O-1(2), may, under the conditions used, preferentially form O-1(2). (C) 2013 Elsevier Inc. All rights reserved.