4 Antimicrobial PDT mediated by fullerenes and their derivatives

Antimicrobial PDT is an emerging alternative to antibiotics motivated by the worldwide growing problems with multi-drug resistant pathogens. The photodynamic activity of chemical compounds such as dyes towards microorganisms was first published at the turn of 20th century and it is based on the concept that a non-toxic dye or PS which, localized preferentially in the microorganism and subsequently activated by low doses of visible light of an appropriate wavelength to be absorbed will generate reactive oxygen species that are toxic to the target microorganisms. Processes in which absorption of light by a PS induces chemical changes in another molecule are defined as photosensitizing reactions. Since around the middle of the last century antibacterial photosensitizing reactions were forgotten because of the discovery of antibiotics and the beginning of the Golden Age of antimicrobial therapy [77]. In the last few decades, however, the relentless worldwide increase in antibiotic resistance amongst bacteria and other infectious microorganisms has led to predictions of untreatable infections caused by “superbugs” [78] and forecasts of “the end of the antibiotic era” [79]. Many researchers are therefore investigating alternative strategies for destroying microorganisms infecting tissues, among which is antimicrobial PDT [80]. PDT employs the combination of a nontoxic PS and visible light that is absorbed by the chromophore to produce long-lived triplet states that can carry out photochemistry in the presence of oxygen to kill cells.