Protoporphyrin IX-mediated PDI against reference strains in TSB medium. The bacterial suspensions were illuminated after dark incubation for 30 min. at 37°C with different concentrations of PpIX (up to 50 μM). PDI was tested against reference strains of S. aureus: RN6390, RN6390sodA, RN6390sodM, RN6390sodAM. Bacteria were illuminated with 12 J/cm2 624 ± 18 nm light, and survival fractions were determined as described in Methods. Values are means of at least three separate experiments.