First introduced in the 1990s to treat cancers, photodynamic therapy (PDT) involves the delivery of lethal drugs to targets (tumours or microbes) by the combination of a light-activated chemical (photosensitizer), light and oxygen.

When illuminated by appropriate wavelengths, the photosensitizer is excited from a lower-energy "ground state" to a higher-energy "triplet state" and can then interact with molecular oxygen in the surroundings to generate reactive oxygen species (ROS). Such ROS, and singlet oxygen in particular, can damage plasma membranes and DNA, which eventually leads to cell death.

The advantage of using photodynamic inactivation for treating and controlling MRSA infections over conventional antimicrobials lies in the fact that MRSA is unlikely to develop resistance to the photochemically induced killing, which, among other ROS, is mediated predominantly by singlet oxygen.

In a recent study published in the journal Nanotechnology, researchers have demonstrated a new type of photosensitizer – Rose Bengal (RB) decorated silica nanoparticles – which can photodynamically inactivate Gram-positive bacteria with high efficiency.

The results show that the RB-decorated nanoparticles are more effective than the RB molecules alone under the same experimental conditions.

The nanoparticle-based photosensitizer has a number of advantages over the free photosensitizing molecules. By concentrating the photosensitizing molecules onto the surface of the nanoparticle, the locally generated singlet oxygen may reach a higher concentration and cause more damage to the target bacteria. In addition, association with the nanoparticles makes the photosensitizing molecules more resistant to photobleaching.