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Molecules 2018, 23(12), 3190; https://doi.org/10.3390/molecules23123190

Inorganic Salts and Antimicrobial Photodynamic Therapy: Mechanistic Conundrums?

1
Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA 02114, USA
2
Department of Dermatology, Harvard Medical School, Boston, MA 02115, USA
3
Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA 02139, USA
4
Laser Research Centre, Faculty of Health Science, University of Johannesburg, Johannesburg, Doornfontein 2028, South Africa
*
Author to whom correspondence should be addressed.
Received: 22 October 2018 / Revised: 15 November 2018 / Accepted: 17 November 2018 / Published: 3 December 2018
(This article belongs to the Special Issue Advances in Photodynamic Therapy 2018)
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Abstract

We have recently discovered that the photodynamic action of many different photosensitizers (PSs) can be dramatically potentiated by addition of a solution containing a range of different inorganic salts. Most of these studies have centered around antimicrobial photodynamic inactivation that kills Gram-negative and Gram-positive bacteria in suspension. Addition of non-toxic water-soluble salts during illumination can kill up to six additional logs of bacterial cells (one million-fold improvement). The PSs investigated range from those that undergo mainly Type I photochemical mechanisms (electron transfer to produce superoxide, hydrogen peroxide, and hydroxyl radicals), such as phenothiazinium dyes, fullerenes, and titanium dioxide, to those that are mainly Type II (energy transfer to produce singlet oxygen), such as porphyrins, and Rose Bengal. At one extreme of the salts is sodium azide, that quenches singlet oxygen but can produce azide radicals (presumed to be highly reactive) via electron transfer from photoexcited phenothiazinium dyes. Potassium iodide is oxidized to molecular iodine by both Type I and Type II PSs, but may also form reactive iodine species. Potassium bromide is oxidized to hypobromite, but only by titanium dioxide photocatalysis (Type I). Potassium thiocyanate appears to require a mixture of Type I and Type II photochemistry to first produce sulfite, that can then form the sulfur trioxide radical anion. Potassium selenocyanate can react with either Type I or Type II (or indeed with other oxidizing agents) to produce the semi-stable selenocyanogen (SCN)2. Finally, sodium nitrite may react with either Type I or Type II PSs to produce peroxynitrate (again, semi-stable) that can kill bacteria and nitrate tyrosine. Many of these salts (except azide) are non-toxic, and may be clinically applicable. View Full-Text
Keywords: antimicrobial photodynamic inactivation; potentiation by inorganic salts; sodium azide; potassium iodide; potassium bromide; potassium thiocyanate; potassium selenocyanate; sodium nitrite antimicrobial photodynamic inactivation; potentiation by inorganic salts; sodium azide; potassium iodide; potassium bromide; potassium thiocyanate; potassium selenocyanate; sodium nitrite
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Hamblin, M.R.; Abrahamse, H. Inorganic Salts and Antimicrobial Photodynamic Therapy: Mechanistic Conundrums? Molecules 2018, 23, 3190.

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