Photons for Therapy: Targeted Photodynamic Therapy for Infected and Contaminated Wounds

Battlefield wounds are frequently contaminated with pathogenic microorganisms present on uniforms and skin. Although the development of serious infections can often be prevented by antibiotics, the rise in worldwide incidence of multiply antibiotic-resistant bacteria necessitates the discovery of alternative methods. In addition, traumatic wounds and burns may contain non-perfused tissue where antibiotics cannot penetrate efficiently. The possibility also exists of the use of biological weapons with unknown antibiotic susceptibility. We report on the use of a targeted polycationic photosensitizer conjugate between poly-L-lysine and chlorin(e6) that can penetrate the Gram (-) outer membrane together with harmless red laser light to kill Escherichia coli and Pseudomonas aeruginosa infecting excisional wounds in mice. We used genetically engineered luminescent bacteria that allowed the infection to be imaged in mouse wounds using a sensitive CCD camera. Wounds were infected with 5x10(exp 6) bacteria, followed by application of the conjugate in solution and illumination with red light. There was a light-dose dependent loss of luminescence as measured by image analysis in the wound treated with conjugate and light, not seen in control wounds. This strain of E. coli is non-invasive and the infection in untreated wounds spontaneously resolved in a few days and all wounds healed equally well showing the photodynamic treatment did not damage the host tissue. P. aeruginosa is highly invasive and mice with photosensitizer alone, light alone or untreated infected wounds all died while 90% of PDT treated mice survived. Wounds treated with PDT healed significantly better than those treated with an alternative antimicrobial (silver nitrate). We then treated mice with an infection caused by Staphylococcus aureus that had been allowed to grow in abscesses below the skin.