A new preclinical approach for treating chronic osteomyelitis induced by Staphylococcus aureus: in vitro and in vivo study on photodynamic antimicrobial therapy (PAmT)

Osteomyelitis is an acute or chronic inflammation in the marrow spaces in the superficial or cortical bone, and can be associated with bacterial or fungal infections. Chronic osteomyelitis represents a major health problem due to its difficult treatment and increased morbidity. Photodynamic antimicrobial therapy (PAmT) is a treatment based on a cytotoxic photochemical reaction in which a bright light produced by a laser system and an active photosensitizer absorbed by cells leads to a process of activation that induces a series of metabolic reactions that culminates a bacterial killing. The aim of the present randomized study was to evaluate, by in vitro and in vivo microbiological analysis, the effects of PAmT on tibial surgical bone defects in rats infected by Staphylococcus aureus using bacterial counts carried out immediately and after 30 days after treatment as outcome measure. In the preliminary in vitro study, a diode laser (λ660 nm; 40 mW; ϕ = 0.4 cm2; 5 or 10 J/cm2) and 5, 10, and 15 μg/mL toluidine blue were tested, and the best parameter was chosen for the in vivo study. The concentration of 5 μg/mL was selected to perform the decontamination of S. aureus-infected tibial bone defects in rats. The findings were subjected to statistical analysis. For all PAmTs groups, with the different concentrations, treatment showed significant reductions (p < 0.001) in the amount of bacteria. The in vivo study PAmT group presented a bacterial reduction of 97.4 % (p < 0.001). The PAmT using toluidine blue was effective in reducing the number of S. aureus in both in vitro and in vivo studies.