Photodynamic inactivation of methicillin-resistant Staphylococcus aureus on skin using a porphyrinic formulation

Staphylococcus aureus is responsible for skin and soft tissue infections that can progress to invasive and life-threatening infections. This bacterium easily acquires antibiotic resistance against all classes of antibiotics. Antimicrobial Photodynamic Therapy (aPDT) seems to be a very a promising alternative for the treatment of localized infections in response to the growing problem of antibiotic resistance, requiring only a photosensitizer, light and oxygen. Thus, the aim of this study was to evaluate the efficiency of a porphyrinic formulation (FORM) as photosensitizer in the photoinactivation of a methicillin-resistant S. aureus (MRSA) strain on skin. Potassium iodide (KI) and iodopovidone (PVP-I) were also tested in combination with FORM as aPDT potentiator agents. The in vitro results showed that FORM was effective to inactivate MRSA. A substantial reduction in the irradiation time, when compared to FORM alone, was observed for FORM + KI and FORM + PVP-I combinations. In the ex vivo assays with porcine skin artificially contaminated with the MRSA strain, reductions in bacterium survival of 3.1 Log₁₀ (CFU) mL⁻¹ were observed with FORM at 50 μM. Although the combined action of FORM + KI and FORM + PVP-I potentiated the aPDT efficacy in vitro, this effect was not observed ex vivo. Overall, the results showed that aPDT using FORM, even without coadjutants, is a promising approach for MRSA inactivation on skin.