The worldwide rise in antibiotic resistance necessitates the development of novel antimicrobial strategies. Although many workers have used
photodynamic therapy (
PDT) to kill bacteria in vitro, the use of this approach has seldom been reported in vivo in animal models of
infection. We have previously described the first use of
PDT to treat excisional
wound infections by Gram-(-) bacteria in living mice. However, these infected
wound models involved a short timespan between
infection (30 min) and treatment by
PDT. We now report on the use of
PDT to treat an established
soft-tissue infection in mice. We used Staphylococcus aureus stably transformed with a Photorhabdus luminescenslux operon (luxABCDE) that was genetically modified to be functional in Gram-(+) bacteria. These engineered bacteria emitted bioluminescence, allowing the progress of the
infection to be monitored in both space and time with a low light imaging charge-coupled device (CCD) camera. One million cells were injected into one or both thigh muscles of mice that had previously been rendered neutropenic by
cyclophosphamide administration. Twenty-four hours later, the bacteria had multiplied more than one hundredfold; poly-
L-lysine chlorin e6 conjugate or free
chlorin e6 was injected into one area of infected muscle and imaged with the CCD camera. Thirty minutes later, red light from a
diode laser was delivered as a surface spot or by interstitial fiber into the
infection. There was a light dose dependent loss of bioluminescence (to <5% of that seen in control
infections) not seen in untreated
infections or those treated with light alone, but in some cases, the
infection recurred. Treatment with conjugate alone led to a lesser reduction in bioluminescence.
Infections treated with free
chlorin e6 responded less well and the
infection subsequently increased over the succeeding days, probably due to
PDT-mediated tissue damage.
PDT-treated infected legs healed better than legs with untreated
infections. This data shows that
PDT may have applications in drug-resistant
soft-tissue infections.