Using Light to Kill MRSA

An old technique called photodynamic antimicrobial therapy (PACT) is being used to kill methicillin-resistant Staphylococcus aureus (MRSA).

Researchers at Queen's University in Belfast (UK; www.qub.ac.uk) say the treatment has the potential to treat MRSA infection in wounds, burns, and leg ulcers. PACT uses a combination of visible light and a photosensitizing drug to cause destruction of microbial cells via singlet oxygen production. Singlet oxygen is a high-energy state of oxygen that kills bacteria by damaging their DNA or their cell membrane.

To test the efficacy of PACT, researchers tested it on two conditions of MRSA growth: in a method known as "suspension,” and also in "biofilms.” In both sets of experiments, bacteria were killed by the photodynamic therapy. To use PACT clinically will require controlling delivery of the photosensitizer into the wound area. A shear-sensitive gel is being developed and evaluated for this role. The gel can be poured into the wound to fill the cavity, but can be removed in one piece. The idea is that the gel would be left in the wound area for a specific time to allow the sensitizers to diffuse out and be taken up by bacteria. The gel would then be removed and light would be directed into the area to activate the photosensitizer and produce the toxic singlet oxygen.

"We are not looking for selectivity to MRSA as this would increase the risk of resistance developing,” explained Dr. Ryan Donnelly, a member of the research team. "What we want is selectivity to bacteria as compared with human cells. PACT offers this selectivity because, over the short time that the gel is in place, the photosensitizer accumulates more readily in bacterial cells than human cells.”




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