Laser destroys bacteria and viruses without using harmful UV radiation

David L. Shenkenberg

A novel laser method could enable safe and effective treatment of drug-resistant viruses and bacteria, including HIV and methicillin-resistant Staphylococcus aureus. Although existing laser treatments such as ultraviolet irradiation bypass the problem of drug resistance, such treatments usually have clinical side effects. For example, ultraviolet radiation is known to cause cancer and can kill healthy cells in the process of destroying pathogenic microorganisms.

Instead of using these traditional techniques, the new method uses vibrations from a femtosecond laser to break the links among the proteins in the protein coats of microorganisms via a process called impulsive stimulated Raman scattering. As a result, the femtosecond laser destroys the protein coat in a process that resembles high-pitched sounds shattering glass. Because femtosecond lasers operate at safe near-infrared wavelengths, this method avoids harmful ultraviolet radiation. It was developed by researchers from Arizona State University in Tempe, from Johns Hopkins Medical Institutions in Baltimore and from the Uniformed Services University of the Health Sciences in Bethesda, Md.

In the top image, a femtosecond laser induces vibrations in the protein coat that surrounds the virus M13 bacteriophage in a process called impulsive stimulated Raman scattering. In the middle image, the amplitude of the vibrations is so large that the weak protein links on the protein coat are broken off, leading to the rupture of the protein coat, as shown in the bottom image, and to the inactivation of the virus. Researchers used this method to inactivate tobacco mosaic virus and E. coli as well.

The investigators tested the ability of femtosecond laser treatment to destroy Escherichia coli and viruses that included M13 bacteriophage and tobacco mosaic virus. They also tested the safety of the treatment on human cells from the Jurkat T-cell line.

The femtosecond laser that the researchers used was a continuous-wave, mode-locked Ti:sapphire laser from Del Mar Photonics Inc. of San Diego, which was pumped with a diode laser from Coherent Inc. of Santa Clara, Calif. They operated the laser at 850 nm because that wavelength is relatively transparent to water, which is prevalent in the human body. It delivered pulses with an average power of ~500 mW at a rate of ∼80 fs. By varying the spot size of the laser with lenses, they achieved high power densities that were used to deactivate the pathogenic microorganisms.

They deactivated M13 bacteriophage and tobacco mosaic viruses at power densities on the order of a few hundred megawatts per square centimeter, and they disabled E. coli at nearly one gigawatt per square centimeter. In contrast, the human cells survived until the power density reached tens of gigawatts per square centimeter. The researchers said that these results suggest that their laser method can deactivate viruses and kill bacteria, while leaving human cells unharmed.

K.T. Tsen, the Arizona State University professor who led the investigation, said that the team currently is investigating the technique for use in destroying HIV and for disinfecting blood supplies or biological materials commonly carried in hospitals. He predicted that it might take two to three years for a product to show up in hospitals.

Journal of Physics: Condensed Matter, Nov. 28, 2007, 472201.