The increase of SodM level was also observed, but only when cells were exposed to externally generated oxidative stress (xanthine/xanthine oxidase) [16]. Summarizing, although we did observe some differences of the basic Sod activity levels in PDI-susceptible vs. PDI-resistant strains, their statistical relevance is not obvious and does not explain the huge differences in PDI-based bactericidal efficacy (Table 2). The reports previously published by our group showed that the bactericidal effect of PpIXArg2-based photokilling was almost completely abolished, when PS was washed away after incubation (before light exposure) [25]. This indicated

that externally generated ROS are responsible for bacterial JNK-IN-8 cell destruction. In regard to our currently presented results we also noticed that some amount of PS enters the cell and influences the transcription of certain genes, eg. sodA and sodM. We observed an increase Cell Cycle inhibitor in sodA and sodM transcript levels but only in 472 and 80/0, PDI-susceptible strains (Table 2). The strains recognized as PDI-resistant, namely 1397 and 2002, did not demonstrate higher sodA nor sodM transcript levels. These results correlate very well with Sod activity measurements observed in these strains. However, Sod activity increase in only susceptible cells proves that this is probably not the only factor

affecting S. aureus vulnerability to porphyrin-based PDI. Conclusions We confirmed in the presented study that the protoporphyrin-based photokilling efficacy is a strain-dependent phenomenon. We showed that oxidative stress sensitivity caused by the lack of both Sod enzymes can be relieved in the presence of Mn ions and partially in the presence of Fe ions. The fact that Sod activity increase Liothyronine Sodium is observed only in PDI-susceptible cells emphasizes that this is probably not

the only factor affecting S. aureus vulnerability to porphyrin-based PDI. Methods Light source BioStimul Lamp which emits polarized (96% level of polarization) monochromatic light (624 nm ± 18 nm) (BIOTHERAPY, Czech Republic) was used for all irradiation experiments. The power of the lamp was measured using a light power meter (model LM1, CARL ZEISS, Jena, Germany). The delivered light energy was approx. 0.2 J/cm2 per minute. Photosensitiser Protoporphyrin IX (MP Biomedicals) stock solution was prepared in 100% dimethyl sulfoxide (DMSO) (Sigma-Aldrich) to the final concentration of 10 mM and kept in the dark at room temperature. Bacterial strains In this investigation we used the reference S. aureus strains: RN6390, RN6390 sodA:: tet (lack of SodA activity), RN6390 sodM::erm (lack of SodM activity), RN6390 sodM::erm sodA:: tet (lack of SodA and SodM activities). These strains were obtained from the collection of Dr. Mark Hart from University of Arkansas, USA [8]. We also investigated eight S. aureus clinical strains isolated from patients from the Provincial Hospital in Gdansk, Poland.