Photodynamic Inactivation of Bacteria

Edgardo   N.   Durantini

Abstract

The emergence of antibiotic resistance amongst pathogenic bacteria has led to a major research effort to find alternative antibacterial therapies. A new promising approach to treat bacterial infections is called bacterial photodynamic inactivation (PDI). This is based in the administration of a photosensitizer, which is preferentially accumulated in the microbial cells. The subsequent irradiation with visible light, in the presence of oxygen, specifically produces cell damages that inactivate the microorganisms. Two oxidative mechanisms can occur after photoactivation of the photosensitizer. In the type I photochemical reaction, the photosensitizer interacts with a biomolecule to produce free radicals, while in the type II mechanism, singlet molecular oxygen, O2(1Δg), is produced as the main species responsible for cell inactivation. Previous investigation showed that porphyrin derivatives can photosensitize the inactivation of various microbial pathogens. In general, the studies show that Gram-positive bacteria are efficiently photoinactivated by a variety of sensitizers, whereas Gram-negative bacteria are usually resistant to the action of negatively charged or neutral agents. The resistance of Gram-negative bacteria to the action of photoactivated sensitizers has been ascribed to the presence of highly organized outer membrane, which hinders the interaction of the photosensitizer with the cytoplasmic membrane and intercepts the photogenerated reactive species. Cationic sensitizers have shown to photoinduce direct inactivation of Gram-negative bacteria without the presence of an additional permeabilization agent. The positive charges on the photosensitizer molecule appear to promote a tight electrostatic interaction with negatively charged sites at the outer surface of the Gram-negative bacteria, increasing the efficiency of the photoinactivation processes. The mainly advantages of PDI are that bacteria can be eradicated in very short time, resistance development in the target bacteria is improbable and damage to adjacent host tissues and disruption of normal microflora can be avoided. This approach is useful to photoinactivate bacteria in a liquid medium and also immobilized on a surface, which allows establishing conditions for the treatment of pathogenic microorganisms growing as localized foci of infection.

Keywords: Photodynamic inactivation, photodynamic therapy, photosensitizer, porphyrin, phthalocyanines, bacteria, Escherichia coli, singlet oxygen