Antibiotics have revolutionized medicine, but antibiotic-resistant bacteria have emerged that can rapidly acquire new defenses, including a strain of Staphylococcus aureus. However, photodynamic therapy (PDT) with a new photosensitizer can kill this superbug, as researchers from the University of Regensburg in Germany have demonstrated. In their experiment, they applied the bacteria to freshly excised skin from a pig and employed the new photosensitizer, which is porphyrin-based. The bacterial strain was methicillin-resistant Staphylococcus aureus, orMRSA. Despite its name, the strain has been reported to develop resistances to several drugs besides methicillin. The scientists induced PDT with a 380- to 480-nm incoherent light source from Waldmann Medizintechnik of Villingen-Schwenningen, Germany.The researchers found that the efficacy of PDT with the new photosensitizer depended upon the concentration of the photosensitizer, the light dose and the duration of incubation. A 10-μM concentration, a light dose of 13.72 J/cm2 and a one-hour incubation period had the greatest effect. Surprisingly, increasing the dose to 100 μM and the incubation time to four hours did not have a greater effect, but the researchers said that the bacteria may have spread deeper than the photosensitizer could penetrate. In fact, images from a Carl Zeiss fluorescence microscope showed that the photosensitizer did not penetrate below the stratum corneum, the most superficial layer of skin. PDT with the photosensitizer greatly reduced the amount of bacteria, and histological results showed that the photosensitizer did not harm surrounding tissue upon irradiation. These findings were reported in the May issue of Photochemical & Photobiological Sciences. The scientists concluded that the new photosensitizer is efficient at killing antibiotic-resistant bacteria and are working to enhance its penetrability.