Although photodynamic therapy (PDT) was discovered over a hundred years ago by its ability
to destroy microorganisms, it has been developed mainly as a cancer therapy. In recent years, due
to the inexorable rise in multi-antibiotic resistant strains of pathogens, PDT is being considered as a
versatile antimicrobial approach to which microbial cells will not be able to develop resistance. The
goal of this review is to survey the different classes of chemical compounds that have been tested as antimicrobial photosensitizers.
Some of these compounds have been known for many years, while others have been rationally designed based
on recently discovered structural principles. Tetrapyrrole-based compounds (some of which are approved as cancer therapies)
that efficiently generate singlet oxygen are more efficient and broad-spectrum when they bear cationic charges, As
the macrocycle structure moves from porphyrins to chlorins to phthalocyanines to bacteriochlorins the long wavelength
absorption moves to the near-infrared where tissue penetration is better. Four main types of natural products have been
tested: curcumin, riboflavin, hypericin and psoralens. Phenothiazinium dyes, such as methylene blue and toluidine blue,
have been tested, and some are clinically approved. A variety of non-phenothiazinium dyes with xanthene, triarylmethane
and indocyanine structures have also been tested. New ring structures based on BODIPY, squaraine and fullerene cages
can also mediate antimicrobial PDT. Finally the process of photocatalysis using titanium dioxide can also have medical
uses. Designing new antimicrobial photosensitizers is likely to keep chemists engaged for a long time to come.
Keywords: Antimicrobial photodynamic therapy, bacteria, bacteriochlorin, chlorin, fullerene, fungi, infection, natural product,
phenothiazinium salt, phthalocyanine, porphyrin, titania photocatalysis, xanthene dye.
Rights & PermissionsPrintExport