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Antimicrobial Agents and Chemotherapy, January 2006, p. 196-203, Vol. 50, No. 1
0066-4804/06/$08.00+0     doi:10.1128/AAC.50.1.196-203.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Phenothiazinium Antimicrobial Photosensitizers Are Substrates of Bacterial Multidrug Resistance Pumps

George P. Tegos^1,2 and Michael R. Hamblin^1,2,3*

Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, Massachusetts,¹ Department of Dermatology, Harvard Medical School, Boston, Massachusetts,² Harvard-MIT Division of Health Sciences and Technology, Cambridge, Massachusetts³

Received 1 July 2005/ Returned for modification 8 September 2005/ Accepted 29 October 2005

Antimicrobial photodynamic therapy (PDT) combines a nontoxic photoactivatable dye, or photosensitizer (PS), with harmless visible light to generate singlet oxygen and free radicals that kill microbial cells. Although the light can be focused on the diseased area, the best selectivity is achieved by choosing a PS that binds and penetrates microbial cells. Cationic phenothiazinium dyes, such as methylene blue and toluidine blue O, have been studied for many years and are the only PSs used clinically for antimicrobial PDT. Multidrug resistance pumps (MDRs) are membrane-localized proteins that pump drugs out of cells and have been identified for a wide range of organisms. We asked whether phenothiazinium salts with structures that are amphipathic cations could potentially be substrates of MDRs. We used MDR-deficient mutants of Staphylococcus aureus (NorA), Escherichia coli (TolC), and Pseudomonas aeruginosa (MexAB) and found 2 to 4 logs more killing than seen with wild-type strains by use of three different phenothiazinium PSs and red light. Mutants that overexpress MDRs were protected from killing compared to the wild type. Effective antimicrobial PSs of different chemical structures showed no difference in light-mediated killing depending on MDR phenotype. Differences in uptake of phenothiazinium PS by the cells depending on level of MDR expression were found. We propose that specific MDR inhibitors could be used in combination with phenothiazinium salts to enhance their photodestructive efficiency.

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* Corresponding author. Mailing address: BAR414, Wellman Center for Photomedicine, Massachusetts General Hospital, 40 Blossom Street, Boston, MA 02114. Phone: (617) 726-6182. Fax: (617) 726-8566. E-mail: hamblin{at}helix.mgh.harvard.edu.

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Antimicrobial Agents and Chemotherapy, January 2006, p. 196-203, Vol. 50, No. 1
0066-4804/06/$08.00+0     doi:10.1128/AAC.50.1.196-203.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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