Bacteria with nanoparticle photosensitizers grow before illumination (left), but are killed after illumination (right) as oxygen is activated. [Source: Peng Zhang]
Bacteria with nanoparticle photosensitizers grow before illumination (left), but are killed after illumination (right) as oxygen is activated. [Source: Peng Zhang]

Researchers from the University of Cincinnati report that they are developing a new method to attack methicillin-resistant Staphylococcus aureus (MRSA) that does not involve antibiotics. Instead, they are using light to activate oxygen, which then wipes out the bacteria. The team says the technique could also be used to treat other microbial infections, and possibly even cancer.

The scientists presented their results in Boston at the National Meeting & Exposition of the American Chemical Society (ACS). 

“Instead of resorting to antibiotics, which no longer work against some bacteria like MRSA, we use photosensitizers, mostly dye molecules, that become excited when illuminated with light,” Peng Zhang, Ph.D., associate professor of chemistry and materials science, University of Cincinnati, says. “Then, the photosensitizers convert oxygen into reactive oxygen species that attack the bacteria.” 

Although other teams have experimented with using these types of photocatalysts to kill bacteria, they did not destroy enough microorganisms to effectively shake off infections, according to Dr. Zhang, who adds that photosensitizers in a molecular form tend to not be corralled enough to do significant damage. In addition, many of them are hydrophobic. This makes it difficult to disperse them in aqueous media where microorganisms typically exist.

To overcome these challenges, Dr. Zhang's group collaborated with Neil Ayres, Ph.D., associate professor, chemistry, University of Cincinnati, and his team. They set out to design a new, water-dispersible, hybrid photosensitizer—one that includes noble metal nanoparticles decorated with amphiphilic polymers to entrap the molecular photosensitizers. 

The team showed that the new nanoparticle photosensitizer was much more effective at killing a variety of bacteria than corresponding formulations that did not contain the metal particles. According to Dr. Zhang, these nanoparticles provide two benefits. The metal has a plasmonic enhancement effect that promotes the generation of more reactive oxygen species, while also concentrating the photosensitizers in one place for a more localized hit to the bacterial cells.

“If you want to attack a castle, and you just let all these people attack individually, it is not very effective. Instead, if you have the same number of people grouped together attacking the castle at one point, it is possible to cause more damage,” explains Dr. Zhang, who has a patent related to the design of hybrid photosensitizers, which can be formulated into a spray or gel.

Dr. Zhang notes that once the spray is developed into a product, medical professionals could put it on any surface and then illuminate it with blue or red light to clean away the bacteria, including MRSA, that may be present. He also says that the method shows promise in direct wound applications to eliminate infection and assist in healing. He has recently performed experiments on laboratory samples of human skin and found that the photosensitizer didn't kill skin cells.

In addition to eradicating MRSA, the nanoparticles are ideal for destroying skin cancer cells, Dr. Zhang says. The nanoparticles perform effectively with the illumination of red light, which has a long wavelength that penetrates deep below the skin, something that's important for a skin cancer treatment. Finally, the nanoparticles have been shown to eliminate nail bed fungus.

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