Potassium selenocyanate potentiate antimicrobial photodynamic inactivation (Conference Presentation)

We previously showed that antimicrobial photodynamic inactivation (aPDI) of Gram-positive and Gram-negative bacteria mediated by the phenothiazinium dye, methylene blue (MB) was potentiated by addition of potassium thiocyanate (10mM). The mechanism was suggested to involve a singlet oxygen mediated reaction with SCN- to form sulfite and cyanide and then to produce sulfur trioxide radical anion. We now report that potassium selenocyanate (KSeCN) (concentrations up to 100 mM) can also potentiate (up to 6 logs of killing) aPDI mediated by a number of different photosensitizers: MB, Rose Bengal, and TPPS4 (as low as 200 nM). When a mixture of selenocyanate with these PS in solution was illuminated and then bacteria were added after the light, there was up to 6 logs killing (Gram-negative > Gram-positive) but the antibacterial species decayed rapidly (by 20 min). Our hypothesis to explain this antibacterial activity is the formation of selenocyanogen (SeCN)2 by reaction with singlet oxygen (1O2) as shown by quenching of 1O2 by SeCN- and increased photoconsumption of oxygen. The fact that lead tetra-acetate reacted with SeCN- (literature preparation of (SeCN)2) also produced a short-lived antibacterial species supports this hypothesis.