Single and combined effects of photodynamic therapy and antibiotics to inactivate Staphylococcus aureus on skin

In this study, the effect of antimicrobial photodynamic therapy (aPDT) and aPDT combined with antibiotics to inactivate Staphylococcus aureus in vitro and ex vivo was compared. The tetracationic porphyrin 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin tetra-iodide (Tetra-Py+-Me) was used to inactivate S. aureus in phosphate buffer solution (PBS) (in vitro) and in pork skin artificially contaminated with S. aureus (ex vivo). The results showed an efficient reduction of 8 log in PBS, after 180 min of irradiation under white light at 4.0 mW cm-2 with Tetra-Py+-Me at 5.0 μM. When aPDT was repeated in the presence of antibiotics, an increased effect was observed with ampicillin at 0.5 and 1.0 μg mL-1 (MIC 0.25 μg mL-1) - the full inactivation (8 log) occurred faster, respectively, after 60 and 30 min of irradiation. In ex vivo, a reduction of ∼4 log of S. aureus was observed after 180 min with 50 μM of Tetra-Py+-Me at 150 mW cm-2, but this efficiency increased significantly with the application of three successive light cycles (reduction to the detection limit ∼6 log). The photoinactivation efficiency observed in ex vivo was also significantly improved when the experiments with 50 μM of Tetra-Py+-Me were repeated in the presence of 5.0 μg mL-1 of ampicillin (inactivation of ∼5.6 log). The results showed that aPDT is an effective approach to control S. aureus infection in skin, inactivating the bacterium to the detection limit after three successive cycles of treatment or after one cycle by using the combination aPDT and ampicillin.