BackgroundThe widespread problem of antibiotic resistance in pathogens such as Staphylococcus aureus has prompted the search for new antimicrobial approaches. In this study we report for the first time the use of a light-activated antimicrobial agent, methylene blue, to kill an epidemic methicillin-resistant Staphylococcus aureus (EMRSA-16) strain in two mouse wound models.ResultsFollowing irradiation of wounds with 360 J/cm2 of laser light (670 nm) in the presence of 100 μg/ml of methylene blue, a 25-fold reduction in the number of viable EMRSA was seen. This was independent of the increase in temperature of the wounds associated with the treatment. Histological examination of the wounds revealed no difference between the photodynamic therapy (PDT)-treated wounds and the untreated wounds, all of which showed the same degree of inflammatory infiltration at 24 hours.ConclusionThe results of this study demonstrate that PDT is effective at reducing the total number of viable EMRSA in a wound. This approach has promise as a means of treating wound infections caused by antibiotic-resistant microbes as well as for the elimination of such organisms from carriage sites.

Staphylococcus aureus is among the most important human pathogens and causes various superficial and systemic infections. The ability of S. aureus to be internalized by, and survive within, host cells, such as keratinocytes, may contribute to the development of persistent or chronic infections and may finally lead to deeper tissue infections or dissemination. To examine the mechanisms of internalization of S. aureus by keratinocytes, isogenic mutants lacking fibronectin-binding proteins (FnBPs), a recombinant protein consisting of the fibronectin-binding domain of S. aureus FnBPs, and an anti-␣5␤1 antibody were used in cocultures with immortalized keratinocytes and primary keratinocytes. We found that internalization of S. aureus by immortalized keratinocytes requires bacterial FnBPs and is mediated by the major fibronectin-binding integrin ␣5␤1. In contrast to internalization by immortalized keratinocytes, internalization of S. aureus by primary keratinocytes could occur through FnBP-dependent and -independent pathways. S. aureus clumping factor B (ClfB), which was recently determined to bind to epithelial cells, was not involved in the uptake of this bacterium by keratinocytes. The identification of an alternate uptake pathway, which is independent of S. aureus FnBPs and host cell ␣5␤1, has important implications for the design of therapies targeted to bacterial uptake by host cells.

Previous studies have shown one of the causative agents of periodontitis, Porphyromonas gingivalis, can be killed by red light in the presence of the light-activated antimicrobial agent toluidine blue O (TBO). The purpose of this study was to determine the effects of irradiating the organism with red light in the presence of TBO on its proteolytic enzyme activity.Suspensions of P. gingivalis were exposed to light with a wavelength of 633 nm in the presence of various concentrations of TBO. Samples were taken at various times and their proteolytic activity determined by assay of azocasein hydrolysis. On exposure to 126 J of red light in the presence of 12.5 µg/ml of TBO the proteolytic enzyme activity was reduced by 100%.The results of this study have shown that the main virulence factor of P. gingivalis, its proteolytic activity, can be inactivated by red light in the presence of TBO. This, together with the known bactericidal activity of the TBO/light combination, suggests that photodynamic therapy may prove important in reducing the effectiveness of P. gingivalis as a periodontopathogen in vivo.