Dr. Manuela Buonanno

Far-UVC Light and the Fight Against Superbugs

Dr. Buonanno is an Associate Research Scientist at the Center for Radiological Research and the Radiological Research Accelerator Facility (RARAF). One of her research interests focuses on characterizing the anti-microbial effectiveness of far-UVC light. The biological studies fall into two categories: 1) Efficacy for killing bacteria and viruses and 2) safety after skin/eye exposure.

Ultraviolet (UV) light is the part of the electromagnetic spectrum generally categorized according to the wavelength range in UVA (315-400 nm), UVB (280-315 nm), and UVC (200-280 nm). Far-UVC light, in particular, refers to wavelengths in the 200-220 nm region. In general, the higher the wavelength the more penetrating the UV light.

Drug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), and airborne-transmitted microbes such as influenza and tuberculosis, together present major health issues both in the developed and the developing world, with major healthcare and economic consequences. While enormous resources have been applied to counter these health problems, effective prevention remains elusive.

UV light is a well-established, highly efficient anti-microbial modality, effective against both bacteria and viruses. However, it is generally not practical to use in scenarios where people are present because UV can cause cataracts and cancer.

We have developed an approach for UV-based sterilization using single-wavelength UVC light to kill microbes, but potentially without harming human cells or tissues. It involves the use of far-UVC radiation generated by inexpensive filtered excimer lamps that emit primarily a single UVC wavelength; in particular, our approach has used a krypton-bromine (Kr-Br) or a krypton-chlorine (Kr-Cl) excimer lamp that produces high-intensity light at 207 nm or 222 nm, respectively. In addition, excimer lamps contain no mercury and are thus environmentally safe compared with conventional germicidal lamps.

The mechanistic background is that far UVC light in the wavelength range of around 200 to 220 nm is strongly absorbed by essentially all proteins, and so its ability to penetrate biological material is very limited. For example, the intensity of 207-nm UV light is reduced by half in about 0.3 μm of tissue, compared with about 3 μm at 250 nm and much longer distances for longer UV wavelengths. The very short half value distance of 207 nm UV light in biological material means that, while it can penetrate bacteria and viruses that are typically smaller than 1 μm in size, it cannot penetrate the human stratum corneum (the outer dead-cell skin layer, thickness 5-20 μm), nor the ocular cornea (thickness ~500 μm), nor even the cytoplasm of individual human cells (Figure 1).

We have shown in vitro that UV light at 207 nm produces no significant biological damage in a human skin 3-D tissue model. Then we extended the 207-nm safety studies in vivo in a hairless mouse skin model using a UV fluence at which 207-nm UV excimer light and 254-nm light from a germicidal lamp are both highly effective for inactivating MRSA, as assessed in our earlier in-vitro bactericidal studies.

Eight relevant cellular and molecular damage endpoints including epidermal hyperplasia, pre-mutagenic UV-associated DNA lesions, skin inflammation, and normal cell proliferation and differentiation were evaluated in mice dorsal skin harvested 48 h after UV exposure. Figure 2 shows the increase in epidermal thickness (left panel) and DNA photodamage (right panel) measured as percentage of cyclobutane pyrimidine dimers (CPD) and 6-4 pyrimidine pyrimidone dimers (6-4 PP) (shown as black nuclei) that are typically induced by UV light exposure.

We found that while conventional germicidal UV (254 nm) exposure produced significant effects for all the studied skin damage endpoints, the same fluence of 207 nm UV light produced results that were not statistically distinguishable from the zero exposure controls. Similar results were obtained with the 222-nm light.

In conclusion, as predicted by biophysical considerations and in agreement with earlier in vitro studies, far-UVC light does not appear to be significantly cytotoxic to mouse skin. These results suggest that far-UVC light could potentially be used for its anti-microbial properties, but without the associated hazards to skin of conventional germicidal UV lamps.

Potentially major applications include in-surgery bacterial control and preventing airborne spread of a variety of viruses and bacteria. Specifically, the many demonstrated anti-microbial applications of germicidal UV lamps, which cannot currently be put into practice when humans are present because of safety considerations, can now be considered as potentially practical even in the presence of humans.

One particular advantage is that UV-mediated bacterial killing is independent of drug resistance so our approach represents a potential methodology for addressing this ever-growing problem.