Photodynamic inactivation of methicillin-resistant Staphylococcus aureus and Escherichia coli: A metalloporphyrin comparison

•Chelation of palladium with photosensitizer had optimal bactericidal activity.•Gram-positive MRSA was more susceptible to PDI compared to Gram-negative E. coli.•Reduced glutathione abrogates PDI activity associated with metalloporphyrins.•Utilizing handheld LEDs for PDI supports its use for self-treatment.

Increasing rates of antibiotic resistance coupled with the lack of novel antibiotics threatens proper clinical treatment and jeopardizes their use in prevention. A photodynamic approach appears to be an innovative treatment option, even for multi-drug resistant strains of bacteria. Three components are utilized in photodynamic inactivation: a photosensitizer, light source, and oxygen. Variations in photosensitizers strongly influence microbial binding and bactericidal activity. In this study, four different cationic metalloporphyrins (Cu2 +, Fe2 +, Pd2 +, Zn2 +) were compared to the free-base ligand 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin regarding their electronic properties and generation of reactive oxygen species upon subsequent 405 nm violet-blue irradiation. Staphylococcus aureus and Escherichia coli were used as representatives of Gram-positive and -negative, respectively, to assess bactericidal effects by the photodynamic process. Bacterial cultures were pre-incubated with porphyrins and exposed to varying doses of 405 nm irradiation (0-30 J/cm2). Metalloporphyrins containing Cu2 + and Fe2 + demonstrated minimal effects on viability. Pronounced bactericidal activity was evident with free-base ligand, Zn2 +, and Pd2 +; though significantly stronger effects were apparent with Pd2 +. Photodynamic killing was directly proportional to reactive oxygen species production post-illumination. These data provide new insight into the influence of metal chelation on photosensitizer activity on bactericidal singlet oxygen production. The strong anti-microbial photodynamic action through the use of a portable light-emitting diode over short time intervals (seconds) provides support for its potential use in self-treatment.