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

Staphylococcus aureus is responsible for skin and soft tissue infections. Having in mind increased antibiotic resistance, in this study the efficacy of antimicrobial photodynamic therapy (aPDT) with a porphyrinic formulation (FORM) as photosensitizer (PS) to photoinactivate methicillin-resistant Staphylococcus aureus (MRSA) on skin was evaluated. Potassium iodide (KI) and iodopovidone (PVP-I) were also tested in combination with FORM as potentiator agents of FORM efficacy. The aPDT protocol was first developed in Phosphate Buffered Saline (PBS, in vitro). Porcine skin was artificially contaminated with MRSA (ex vivo) and treated with FORM, FORM + KI or FORM + PVP-I under white light. 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. On skin, reductions in MRSA survival of 3.1 Log(10) colony forming units (CFU) mL(-1) were observed with FORM at 50 mu M. Although the combined action of FORM + KI and FORM + PVP-I potentiated the aPDT efficacy in vitro, this 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.