The growing rise in the development of multidrug-resistant strains of bacteria towards conventional antibiotics necessitates exploring alternative techniques such as antimicrobial photodynamic inactivation (aPDI). aPDI relies on the activation of a photosensitizer (PS) by a specific wavelength of light with the production of excess reactive oxygen species (ROS), which have the ability to successfully eradicate a wide range of human pathogens like bacteria (either Gram-positive Gram (+) or Gram-negative Gram (-)), fungi, protozoa, parasites, viruses, and even bacterial biofilms. One of the notable advantages of aPDI is that it doesn't lead to bacterial resistance or be affected by the already established resistance to antibiotics.
The characteristics of the photosensitizer used have a major impact on how effective aPDI is. The best PS for selective aPDI is thought to have a strong positive charge, be safe in the dark, and produce a large quantity of ROS when activated by red light. Various PSs, either natural or synthetic, have been proven effective in aPDI. The synthetic dye methylene blue and the natural PS curcumin have been extensively explored. Moreover, tetrapyrrole structures like porphyrins and phthalocyanines have been extensively investigated because they are easily chemically modified. Nanocarriers played a significant role in aPDI, as some nanocarriers function as PSs by themselves, like fullerenes, while others bind PS to their surfaces or embed it within their matrix. Nanocarriers have been demonstrated to enhance the antibacterial activity of the PS, protect it, and improve its delivery to the target site.