The phototoxicity of phenothiazinium derivatives against Escherichia coli and Staphylococcus aureus

Phenothiazinium dyes, and derivatives, were tested for toxicity to Escherichia coli and Staphylococcus aureus. The dyes were generally lipophilic (log P > 1) and showed inherent dark toxicity (minimum lethal concentrations: 3.1-1000 muM). Dye illumination (total light dose of 3.15 J cm(-1) over 30 min) led to up to eight-fold reductions in minimum lethal concentrations. Most of the illuminated dyes showed significant relative singlet oxygen yields (Phi'(Delta): 0.18-1.35) suggesting a type 11 mechanism of generating a phototoxic response. Although generally up to six-fold more effective against S. aureus, the dyes tested efficiently killed E coli and may be of particular use in combating Gram-negative pathogens. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.