Protoporphyrin IX-mediated PDI against clinical strains. 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 clinical S. aureus strains: MRSA, MSSA. Bacteria were illuminated with 12 J/cm2 624 ± 18 nm light, and survival fractions were determined as described in Methods. Values are means of three separate experiments, and bars are SD.