A technique that has been around for more than one hundred years is being used in new research as a way of killing methicillin-resistant Staphylococcus aureus (MRSA) using light. The research to be reported today at the British Pharmaceutical Conference in Manchester, is being undertaken by researchers from Queen's University Belfast.
The treatment, called photodynamic antimicrobial therapy (PACT) has the potential to treat MRSA infection in wounds, burns and leg ulcers. PACT uses a combination of visible light and a photosensitising 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).
Although photodynamic therapy is groundbreaking in the current age - the process has been around for over a century. Dr Ryan Donnelly, from the Belfast team, explains: "One hundred years ago, people looked at killing bacteria by photosensitising them. The method fell out of favour with the development of antibiotics. But with the present problems of antibiotic resistance, there is interest in alternative antimicrobial therapy."
Dr Donelly says that PACT may be the answer to antiobiotic resistance. "The beauty of PACT is that, because singlet oxygen is a non-specific oxidising agent and is only present during illumination, it is highly unlikely that a population of cells would become resistant."
To test the efficacy of PACT, it was tested on two conditions of MRSA growth: in a method known as 'suspension' and also in 'biofilms'. The latter was a worst-case scenario as biofilms are highly resistant to antibiotics. In both sets of experiments, bacteria were killed by the photodynamic therapy.
To use PACT clinically will require controlling delivery of the photosensitiser into the wound area. A shear-sensitive gel is being developed and evaluated by Dr Paul McCarron, a second member of the PACT team, as an innovative drug delivery system. 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 sensitiser 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 photosensitiser and produce the toxic singlet oxygen.
PACT should kill any microbial organisms in the wound area. Dr Donnelly commented: "We are not looking for selectivity to MRSA as this would increase the risk of resistance developing. 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 photosensitiser accumulates more readily in bacterial cells than human cells."