Antimicrobial photodynamic inactivation is potentiated by the addition of selenocyanate: Possible involvement of selenocyanogen?

Abstract

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 the addition of potassium thiocyanate (10 mM). 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 (concentrations up to 100 mM) can also potentiate (up to 6 logs of killing) aPDI mediated by a number of different photosensitizers (PS): MB, rose bengal and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin dihydrochloride (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 of killing (Gram-negative > Gram-positive) but the antibacterial species decayed rapidly (by 20 minutes). Our hypothesis to explain this antibacterial activity is the formation of selenocyanogen (SeCN) by reaction with singlet oxygen ( O ) as shown by quenching of O by SeCN and increased photoconsumption of oxygen. The fact that lead tetraacetate reacted with SeCN (literature preparation of (SeCN) ) also produced a short-lived antibacterial species supports this hypothesis.

Comparison of thiocyanate and selenocyanate for potentiation of antimicrobial photodynamic therapy

“…Next, we wanted to investigate whether any bactericidal compound was formed by the action of aPDT‐induced ROS (singlet oxygen) on the salt. We had previously shown that in the case of KSeCN this was indeed the case, but that the antibacterial activity observed when the cells were added after light, decayed in a short period . For this experiment, we only used MB, as the PS as this was the only one that had been shown to be potentiated by both salts (KSCN and KSeCN) (Figure ).…”

“…We had previously shown that, Pb(OAc) 4 combined with KSeCN, could synergistically kill bacteria . The reason for choosing Pb(OAc) 4 was that it is mentioned as a literature synthetic procedure for the production of selenocyanogen (SeCN) 2 from KSeCN .…”

“…This strategy can be applied to gram‐positive bacteria, gram‐negative bacteria and also to fungi, and is effective in animal models of localized infections . A variety of different salts can be employed, including potassium iodide , potassium bromide , sodium azide , potassium thiocyanate (KSCN) and potassium selenocyanate (KSeCN) . Potassium iodide, potassium bromide and KSCN are nontoxic; KSeCN has low toxicity, while sodium azide has moderate toxicity.…”

“…We had previously shown that aPDT mediated by methylene blue (MB) could be potentiated by addition of 10mM KSCN . We have also previously reported that KSeCN (at concentrations up to 100 mM) could potentiate aPDT mediated not only by MB, but also by Rose Bengal (RB) and also by 5,10,15,20‐tetrakis(4‐sulfonatophenyl)porphyrin dihydrochloride (TPPS4). We recently reported that the supposedly anionic porphyrin TPPS4 behaved more like a cationic porphyrin, and functioned as a surprisingly effective antimicrobial PS especially when potentiated by potassium iodide (KI).…”

Can light‐based approaches overcome antimicrobial resistance?

“…We then tested another different pseudohalide salt which is isoelectronic with KSCN, namely potassium selenocyanate, KSeCN (Huang et al, ). KSeCN at concentrations up to 100 mM could potentiate aPDI giving up to 6 logs of extra killing of E. coli and MRSA.…”