Chapter 3: Antimicrobial properties of hydroxyxanthenes

Hydroxyxanthenes are commonly used as dyestuffs in the food, cosmetics, and textile industries. These compounds also have medical applications due to their unique staining and fluorescent properties. The halogenated hydroxyxanthenes exhibit antimicrobial properties that may be useful for reducing or eliminating bacterial pathogens from a variety of environments, including drinking water and food products. Antimicrobial characteristics of Eosin, Erythrosine, Phloxine, and Rose Bengal have been known for many years, but their application as antimicrobial agents has been limited primarily to selective agents in microbiological growth media. The primary mechanism of bacterial inactivation by hydroxyxanthenes is photooxidation. When halogenated hydroxyxanthenes are photooxidized, a variety of compounds are produced (e.g., singlet oxygen, superoxide anion and other radicals); these exhibit toxicity to the microbial cell. Gram-positive bacteria are particularly sensitive to inactivation by photooxidation of halogenated hydroxyxanthenes; however, different species vary in their sensitivity. Gram-negative bacteria are inherently resistant to inactivation by these compounds due to the barrier properties of the outer membrane that prevent the necessary localization. Treatments such as chelation and ultrahigh pressure may be used to destabilize the outer membrane leading to sensitivity of Gram-negative bacteria to halogenated hydroxyxanthenes.