Bactericidal effect of photodynamic inactivation against methicillin-resistant and methicillin-susceptible Staphylococcus aureus is strain-dependent

Staphylococcus aureus is one of the most important etiological factors responsible for nosocomial infections. Some of them may be life-threatening, especially in the case of immuno-compromised patients, causing bacteremia, endocarditis, sepsis or toxic-shock syndrome. Their multiresistance to antibiotics produces many therapeutic problems, and for this reason the development of a method alternative to antibiotic therapy is needed. It seems that photodynamic inactivation (PDI) may be an effective and alternative therapeutic option against both methicillin resistant (MRSA) and methicillin sensitive (MSSA) S. aureus strains. The aim of this study was to analyze the bactericidal effect of the PDI against 40 clinical MRSA and 40 MSSA clinical strains that were isolated from patients hospitalized in the Provincial Hospital in Gdansk. The ATCC strain 25904 has been used as a reference. Photodynamic inactivation by means of protoporphyrin diarginate as a photosensitizer was examined. It was observed that the bactericidal effect of the PDI was strain-dependent and ranged from 0 to 3 log10-unit reduction in viable counts. The mechanism underlying such a phenomenon is still not understood. Nevertheless, the correlation between the biofilm production ability and different strains response to PDI has been observed.