Bactericidal effects of photoactivated porphyrins--an alternative approach to antimicrobial drugs.

Photoactivated porphyrins display a potent cytotoxic activity towards a variety of Gram positive bacteria, mycoplasma and yeasts, but not Gram negative cells. The prerequisite for photosensitization of a microbial cell is the binding of porphyrin to the cytoplasmic membrane in a pH-dependent manner. On illumination, the membrane bound, and possibly, cytoplasmic porphyrin molecules generate singlet oxygen and radicals which sensitize biomolecules and lead to cell death. The immediate inhibition of cell growth on photodynamic treatment is accompanied by alterations in cell wall and membrane synthesis, leading to the formation of large mesosomes adjacent to the unaccomplished septa. Hemin bound to microbial cells exerts cytotoxic activity by peroxidative and oxidative reactions independent of light. Future research in the field may enhance the possibility of using porphyrin photosensitization for treatment of microbial infections. Such clinical use will be unrelated to the antibiotic resistance of the pathogen. Resistance of Gram negative bacteria to porphyrin photosensitization is the main impediment to its use as a broad spectrum antibacterial method.