207-nm UV Light - A Promising Tool for Safe Low-Cost
Reduction of Surgical Site Infections. I: In Vitro Studies
Manuela Buonanno1, Gerhard Randers-Pehrson1, Alan W. Bigelow1, Sheetal Trivedi2, Franklin D. Lowy2,
Henry M. Spotnitz3, Scott M. Hammer2, David J. Brenner1*
1 Center for Radiological Research, Columbia University Medical Center, New York, New York, United States of America, 2 Division of Infectious Diseases, Department ofMedicine, Columbia University Medical Center, New York, New York, United States of America, 3 Department of Surgery, Department of Medicine, Columbia University
Medical Center, New York, New York, United States of America

Abstract
Background: 0.5% to 10% of clean surgeries result in surgical-site infections, and attempts to reduce this rate have had
limited success. Germicidal UV lamps, with a broad wavelength spectrum from 200 to 400 nm are an effective bactericidal
option against drug-resistant and drug-sensitive bacteria, but represent a health hazard to patient and staff. By contrast,
because of its limited penetration, ,200 nm far-UVC light is predicted to be effective in killing bacteria, but without thehuman health hazards to skin and eyes associated with conventional germicidal UV exposure.
Aims: The aim of this work was to test the biophysically-based hypothesis that ,200 nm UV light is significantly cytotoxic tobacteria, but minimally cytotoxic or mutagenic to human cells either isolated or within tissues.
Methods: A Kr-Br excimer lamp was used, which produces 207-nm UV light, with a filter to remove higher-wavelengthcomponents. Comparisons were made with results from a conventional broad spectrum 254-nm UV germicidal lamp. First,
cell inactivation vs. UV fluence data were generated for methicillin-resistant S. aureus (MRSA) bacteria and also for normal
human fibroblasts. Second, yields of the main UV-associated pre-mutagenic DNA lesions (cyclobutane pyrimidine dimers
and 6-4 photoproducts) were measured, for both UV radiations incident on 3-D human skin tissue.
Results: We found that 207-nm UV light kills MRSA efficiently but, unlike conventional germicidal UV lamps, produces little
cell killing in human cells. In a 3-D human skin model, 207-nm UV light produced almost no pre-mutagenic UV-associatedDNA lesions, in contrast to significant yields induced by a conventional germicidal UV lamp.
Conclusions: As predicted based on biophysical considerations, 207-nm light kills bacteria efficiently but does not appear tobe significantly cytotoxic or mutagenic to human cells. Used appropriately, 207-nm light may have the potential for safely
and inexpensively reducing surgical-site infection rates, including those of drug-resistant origin.
Citation: Buonanno M, Randers-Pehrson G, Bigelow AW, Trivedi S, Lowy FD, et al. (2013) 207-nm UV Light - A Promising Tool for Safe Low-Cost Reduction of
Surgical Site Infections. I: In Vitro Studies. PLoS ONE 8(10): e76968. doi:10.1371/journal.pone.0076968
Editor: Michael Hamblin, MGH, MMS, United States of America
Received July 29, 2013; Accepted September 5, 2013; Published October 15, 2013
Copyright: ß 2013 Buonanno et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This was funded through internal funding at Columbia University Medical Center. The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: djb3@columbia.edu

drug resistance is the use of germicidal UV lamps [8,9], because
UV light is generally equi-effective at inactivating drug-resistantbacteria compared with wild-type strains [10,11]. In fact studies ofsurgical wound irradiation with conventional germicidal UV
lamps have shown great promise, with UV fluences corresponding
to 4 to 5 logs of MRSA cell kill resulting in significant decreases in
SSI rates [8]. However UV radiations in the wavelength range
emitted by a germicidal lamp are a human health hazard, causing
roughly equal levels of biological damage in human cells as well asbacteria [12–14], and are clearly linked with skin cancer [15,16]
and cataract induction [17,18]. These health hazard to humanskin and eyes necessitate the use of cumbersome protective
clothing, hoods and eye shields for the surgical staff and the patient[8,19], and this has prevented widespread use of the UV woundirradiation technique during surgery.

Introduction
Depending on the procedure, between 0.5% and 10% of all
clean surgeries in the US, corresponding to about 275,000 patients
per year, result in surgical site infections (SSI) [1–3]. Patients whodevelop SSI are 60% more likely to spend time in an ICU, are 5
times as likely to be readmitted, have a mortality rate twice that of
non-infected patients, have an average of 7 days additional lengthof hospital stay [4], and have roughly double the total healthcare
costs compared with patients without SSI [5]. The annual numberof deaths in the US attributed to SSI has been estimated at 8,200[3], with annual patient hospital costs between $3 and $10 billion[6].
A key issue contributing to the extent and severity of the SSI
problem is the prevalence of drug-resistant bacteria such as MRSA
[7]. One approach which in principle overcomes the problem of
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207-nm UV for Reducing Surgical Site Infections

The spectrum of wavelengths emitted by a conventional
mercury germicidal UV lamp is broad, covering the UVC, UVB
and UVA ranges from about 200 nm to more than 400 nm, withthe dominant bactericidal effect due to UVC around 254 nm.
Here we suggest and provide evidence here that there exists a
narrow wavelength window in the far-UVC region, around200 nm, in which bacteria are efficiently killed, but which
produces far less cytotoxic or mutagenic damage to human cells.
The proposed bactericidal application of 207-nm UV light in
the presence of humans is based on the fact that UV light at awavelength of around 200 nm is very strongly absorbed byproteins (particularly through the peptide bond) and otherbiomolecules [20,21], so its ability to penetrate biological material
is very limited. Thus, for example the intensity of 200-nm UV lightis reduced by half in only about 0.3 mm of tissue, compared with
about 3 mm at 250 nm and much longer distances for higher UV
wavelengths [22,23]. This phenomenon is quantified in Figure 1[20,24]. By contrast, 200-nmUV light is only minimally absorbed
in water [25].
As we now discuss, this very limited penetration of ,200-nm
UV light in biological material is significant here, both at thecellular and at the tissue level:
At the cellular level, bacteria are, of course, much smaller thanalmost any human cell. Typical bacterial cells are less than 1 mm
in diameter [26,27], whereas typical human cells range indiameter from about 10 to 25 mm [26]. It follows that ,200-nm
UV light can penetrate throughout typical bacteria, but cannot
penetrate significantly beyond the outer perimeter of thecytoplasm of typical human cells - and will be drasticallyattenuated before reaching the human cell nucleus. By contrast
higher wavelength light from a conventional germicidal lamp canreach human cell nuclei without major attenuation (see Figure 1).
Based on these biophysical considerations, while radiation from a
conventional UVC lamp would be expected to be cytotoxic andmutagenic to both bacteria and human cells (which is indeed thecase [12–14]), ,200 nm UV light would be expected to becytotoxic to bacteria but very much less cytotoxic or mutagenic tohuman cells. The current study is designed to test this hypothesis.
At the tissue level, the two organs at major risk from UV light
are the skin and the eye [15–18]. The key skin cells at risk from
UV exposure, such as basal cells and melanocytes, are locatedwithin the epidermis below the stratum corneum which is a 5 to
20 mm thick layer of dead non-nucleated cells [28]. Given its very
limited penetration (half-value thickness ,0.3 mm, as discussed
above), ,200-nm light will thus have minimal ability to penetrate
the stratum corneum and reach the underlying key skin cells suchas basal cells or melanocytes.
Considering now the eye, the most important target from theperspective of UV risk is the lens [17,18]. The lens is located distal
to the cornea, which is sufficiently thick (,500 mm [29],) such thatpenetration of ,200-nm light through the cornea to the lenswould be essentially zero [30]. We also need to consider effects on
the cornea from the perspective of photokeratitis, though any
protective device against eye splash, which is now almost universal
amongst surgical staff, would be expected to fully protect thecornea from 207-nm UV exposure.
Overall, these considerations suggest that irradiating the surface
of a wound area with ,200-nm far-UVC light during the courseof a surgical procedure may provide all the bactericidal advantagesof conventional UV germicidal lamps, while potentially being safefor patient and staff, and requiring no protective clothing. It is thegoal of this paper to provide a first test of the hypothesis of theefficacy and safety of ,200-nm light, using pre-clinical in-vitro
models of MRSA, human cells and human skin tissue. Studies withanimal models are the logical next step.
This approach is made practical by the development [31] of a
UV lamp that emits at close to a single wavelength, in this casearound 207 nm, defined by the gas mixture it contains; this is incontrast to conventional germicidal mercury UV lamps whichemit over a broad range of wavelengths. Such single-wavelength
lamps, based on UV emitted from an excited molecule complex(an exciplex), are called excimer lamps, or excilamps [32]. In the
wavelength region of interest here, excimer lamps are available
containing argon-fluorine, krypton-bromine, or krypton-chlorine
mixtures, which respectively produce far-UVC light at 193, 207
and 222 nm [32]. Excimer lamps are small, rugged, inexpensive,
sufficiently intense, and long-lived (10,000 to 100,000 h) [32–34];
several studies [35–38] have been published on the bactericidal
properties of far-UVC excimer lamps, but the suggestion that,200 nm wavelength UV is differentially cytotoxic to bacteria
relative to human cells is new.
Here we use a filtered excimer lamp that produces essentiallymonoenergetic UV light at 207 nm. We describe measurements ofcell killing by 207-nm light, and also by a conventional 254-nm
germicidal UV lamp, both for methicillin-resistant Staphylococcus
aureus (S. aureus) cells (MRSA) and also for normal human skin cells.
We further describe measurements of typical UV-induced premutagenic DNA lesions in a 3-D human skin model, again bothfrom 207-nm light and from a conventional broad spectrum
germicidal UV lamp.

Materials and Methods
Production and Characterization of Monoenergetic UVLight at 207 nm

Figure 1. Mean wavelength-dependent UV absorbance coefficients, averaged over published measurements for eightcommon proteins. Data derived from Ref [24] for human serum
albumin, horse hemoglobin, human immunoglobulin-c, lysozyme,
bovine serum albumin, bovine a-chymotrypsinogen, bovine immunoglobulin-c and bovine ribonuclease A. Typical standard deviations of
the measurements across these eight proteins [24] are shown.
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Excimer lamps (excilamps) [31,32] produce a high-intensity
spatially-broad UV beam, primarily at a single-wavelength. The
excimer lamp used here was based on a krypton-bromine (Kr-Br)
gas mixture, which emits principally at 207 nm. The lamp (HighCurrent Electronics Institute, Tomsk, Russia [32]) was air-cooled
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and has a 7,500-mm2 exit window. For comparison, studies werealso carried out with a conventional broad-spectrum low-pressure
mercury germicidal lamp (Sankyo Denki G15T8, Japan), with apeak emission at 254 nm.
A UV spectrometer (Photon Control, BC, Canada) sensitive in awavelength range from 200 nm to 400 nm was used to measure
the wavelength spectra emitted by the excimer lamp, and a
deuterium lamp standard with a NIST-traceable spectral irradiance (Newport Corp, Stratford, CT, USA) was used to calibrate
the UV spectrometer and to estimate absolute photon fluences. A
UVX radiometer with a short wavelength sensor (Upland, CA,
USA) was used to measure the fluence rate from the conventional
germicidal lamp.
Figure 2 shows measured spectra emitted from the Kr-Br
excimer lamp. As well as the main excimer emission at 207 nm,
the lamp also emits lower fluences of higher wavelength light,
particularly around 228 and 293 nm. These higher-wavelengthcomponents are undesirable as they are more penetrating (seeFigure 1) and therefore potentially more harmful to human cells.
We used a custom bandpass filter (Omega Optical, Brattleboro,
VT, USA) to remove all but the dominant 207-nm wavelength
emission. Using this filter, the system effectively delivered only acharacteristic single wavelength of 207 nm, as shown in the inset
to Figure 2.
The UV fluence rates at the target cell location were determinedby the lamp intensities and the lamp-target distance. The results
shown here used a lamp-target distance of 400 mm, and measured
fluence rates at the target from the unfiltered and filtered Kr-Br
lamp were respectively 0.33 and 0.044 mW/cm2; at the same
location, the fluence rate from the conventional UV mercury lamp
was 0.45 mW/cm2. These fluence rates could be simply adjusted
up or down by changing the source-target distance. Depending on
the desired fluence, the fluence rates used here resulted in
irradiation times from tens of seconds to tens of minutes. Earlier
studies reported in the literature showed that, for a given UV
fluence, UV inactivation of S. aureus is independent of fluence rate/irradiation time over irradiation times from a few seconds to a few
hours [39].

Figure 3. Cell killing of MRSA and human cells, induced by UVfrom a conventional germicidal lamp. Cell killing is shown relative
to zero-fluence controls, expressed as surviving fraction, or as logs ofcell kill (-log10[surviving fraction]), produced by different fluences of UV
exposure from a conventional broad-spectrum germicidal UV lamp
(peak emission 254 nm). Data are shown for methicillin resistant S.
aureus cells ( , MRSA strain USA300) and for AG1522 normal humanfibroblasts (m).
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MRSA Cell Survival Assay
Due to their clinical relevance [40,41], we chose methicillinresistant S. aureus (MRSA) for these initial studies. The strain
chosen was USA300 (multilocus sequence type 8, clonal complex
8, staphylococcal cassette chromosome mec type IV), originallyencountered in the community, but now a major cause of invasive
nosocomial infections [42].
Survival of MRSA was assessed using the standard colony
forming unit (CFU) assay: Fresh colonies of S. Aureus were
inoculated into tryptic soy broth (Thermo Scientific, Pittsburgh,
PA, USA) and grown overnight at 37uC. The culture was thenresuspended in fresh broth and grown to mid-log phase for 3 h.
Bacteria were collected by centrifugation, washed, resuspended inbroth, and adjusted to an optical density at 600 nm of 0.5. Serial
dilutions were performed in HBSS (Hanks’ balanced salt solution),to yield initial bacterial densities from 108 to 1021 CFU/ml,
depending on the planned UV fluence. 50 ml of the desired
dilution was then spread onto standard 100-mm diameter
prepared media plates (containing a substrate of tryptic soy agar
with 5% sheep blood; BD Diagnostic System, Sparks, MD, USA)using a glass spreader, resulting in a bacterial monolayer on top of
the substrate. Almost all the HBSS used in the spreading was
rapidly absorbed into the agar substrate underneath the cells; wemeasured the average depth of the residual HBSS covering thebacterial monolayer at the time of irradiation, which was,0.01 mm. We confirmed that this residual HBSS would not
significantly attenuate the 207 nm UV light, through directtransmission measurements of 207 nm UV through the HBSS,
which was .99.9% after 2 mm of HBSS, and therefore..99.9% after 0.01 mm of HBSS.
Three media plates were used for each fluence point, and theplates were irradiated from above for different time intervals,
depending on the required fluence. The plates were thenincubated overnight at 37uC, with CFU colony counting
performed the following day. Bacterial inactivation was expressedas the proportional reduction in CFU/mm2, relative to shamirradiated zero-fluence controls.

Figure 2. Non-filtered and filtered measured emission spectrafrom a Kr-Br excimer lamp (main peak 207 nm).
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with water and covered with methyl green counterstain solution atroom temperature for 3 min. The samples were dipped in 100%
EtOH twice and into xylene. Finally, cover slips were mounted oneach sample with mounting medium (Vectashield, Burlingame,
CA, USA). CPD- and 6-4PP positive cells were scored in a total ofnine randomly chosen locations in the epidermis from threedifferent tissue samples.

Normal Human Cell Survival Assay
Normal human diploid skin fibroblasts (AG1522) were obtained
from the American Type Culture Collection (Manassas, VA,
USA). Cells at passage 10–12 were grown in Eagle’s Minimum
Essential Medium (CellGro, Manassas, VA, USA) supplemented
with 12.5% heat-inactivated (56uC, 30 min) fetal calf serum,
200 mM L-alanyl-L-glutamine, 100 U/ml penicillin and 100 mg/
ml streptomycin (Sigma-Aldrich Corp., St. Louis, MO, USA). The
cells were maintained at 37uC in a humidified incubator with 5%
CO2 in air. Cells were seeded in glass chambers (Nalge-Nunc LabTek, Rochester, NY, USA) at a density to reach confluence about
48 h before exposure. Under these conditions, 90–98% of the cells
were in G0/G1 phase of the cell cycle, as determined by tritiatedthymidine incorporation and flow cytometry.
Cell survival was determined by the standard colony formation
assay [43]. Briefly, cells were trypsinized within 5–10 min of
exposure, suspended in growth medium, counted, diluted andseeded in 100-mm dishes with numbers resulting in ,100
clonogenic cells per dish. Three replicates were done for eachdata point. After an incubation period of 12 to 14 days, the cells
were rinsed with phosphate buffered saline (PBS), fixed in 95%
ethanol and stained with crystal violet. Macroscopic colonies withmore than 50 cells were scored as survivors. Measured survival
values were corrected for the system plating efficiency whichranged from 20% to 30% as measured with sham-irradiatedcontrols.

Results
1 Cell Survival
We measured cell survival in MRSA (strain USA300) and innormal human skin fibroblasts (AG1522) exposed to different
fluences of UV light generated by a 254-nm broad-spectrum
germicidal UV lamp (Figure 3), and by 207-nm UV light from afiltered Kr-Br excimer lamp (Figure 4).
As illustrated in Figure 3, the conventional broad-spectrum
germicidal UV lamp (broad spectrum, peak 254 nm) efficiently
kills MRSA cells but, as expected, is almost equally efficient atkilling human cells. By contrast, as illustrated in Figure 4, at the
fluences studied here, the 207-nm excimer lamp provided up to
four logs of MRSA cell killing but produced little cell killing inhuman cells – quantitatively about a 5,000 fold differential in cellkilling between MRSA and human cells.
For example, four logs of MRSA cell killing (cell survival level of1024) required fluences of 35 mJ/cm2 of 254-nm (broad-spectrum)
UV and 135 mJ/cm2 of 207-nm light; these fluences respectively
produced cell killing in human cells of 261024 (254 nm) and
261021 (207 nm). Thus for the same level of MRSA killing, the207-nm excilamp produces about 1,000-fold less killing in humancells compared to a conventional germicidal lamp.
This major differential in human cell killing between broadspectrum 254 nm UV and 207 nm light is consistent with thatobserved by Kochevar et al. [47,48], who compared the effects ofbroad spectrum 254-nm UV with 193-nm laser light; consistent
with the hypothesis in the current work, they attributed the lowlevel of human cell killing by 193-nm laser light to non-DNA based
membrane damage.

Measurement of UV-associated Pre-mutagenic DNA
Lesions in 3-D Human Skin Model
The 3-D human skin model used was a full skin thickness
construct which recapitulates all the key components of humanskin, within the epidermal (including the stratum corneum) andthe dermal layers [44]. The construct (EpiDerm-FT, purchasedfrom MatTek Corp., Ashland, MA, USA) consists of organizedbasal, spinous, granular, and cornified layers analogous to thosefound in vivo.
The two pre-mutagenic DNA lesions assayed in the epidermis
were cyclobutane pyrimidine dimers (CPDs) and pyrimidinepyrimidone 6-4 photoproducts (6-4PPs), which are generally
considered the major pre-mutagenic DNA lesions linked with
UV-induced skin cancer [16].
The two lesions were assayed and scored using standard
immunohistochemical approaches, following the protocols described, for example, by Stolper et al [45] in Epiderm-FT
constructs and by Qin et al [46]. In brief, after UV irradiation,
the EpiDerm-FT cultures were fixed in 10% neutral bufferedformalin (overnight, room temperature), paraffin embedded,
sectioned with a microtome, and stained with hematoxylin andeosin. 5 mm paraffin-embedded cross sections mounted on glass
slides were deparaffinized and heat-mediated antigen retrieval was
conducted. Slides were incubated in PBS followed by 0.05 N HCl
at 4uC for 5 min before being washed sequentially in PBS, 50%
EtOH and 70% EtOH, 0.15 N NaOH, 70% EtOH, 50% EtOH,
and PBS for 2 min each at room temperature. Slides were blocked
in 1% bovine serum albumin in PBS for 1 h followed by mouseanti-CPDs (Kamiya Biomedical Company, Seattle, WA, USA) or
mouse anti-6-4PPs (Cosmo Bioscience USA, Carlsbad, CA, USA)
1:50 in blocking buffer for 1 h at room temperature. Samples were
washed in PBS (3 times, 5 min each) and incubated for 1 h with abiotinylated secondary antibody (1:200 in PBS) followed by a
5 min wash in PBS, streptavidin-HRP (Sigma-Aldrich Corp., St.
Louis, MO, USA) for 30 min, and finally DAB substratechromogen solution for 15 min as specified by EMD Millipore
protocol (Billerica, MA, USA). The samples were then washed
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Figure 4. Cell killing of MRSA and human cells, induced by 207nm UV light. Cell killing is shown relative to zero-fluence controls,
expressed as surviving fraction or as logs of cell kill (-log10[surviving
fraction]), produced by different fluences of UV light from a filtered 207nm Kr-Br excimer lamp. Data are shown for methicillin resistant S.
aureus cells ( , MRSA strain USA300) and for AG1522 normal humanfibroblasts (m).
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lamps are almost as efficient at killing human cells. Quantitatively,
for a four-log level of MRSA killing, 207-nm UV light produces
about 1,000-fold less human cell killing than a conventional
germicidal UVC lamp.
In terms of the safety of 207-nm UV light, the lack of inductionof typical UV-associated pre-mutagenic DNA lesions in theepidermis of a 3-D skin model is again consistent with biophysicalexpectations based on the limited penetration of 207-nm UV light,
and consistent with earlier studies using 193-nm laser light [23].
Whilst other UV wavelengths have been suggested as potentiallysafe for human exposure, in particular 405 nm [49] and 254 nm[50], no mechanisms have been proposed for a differential toxic
effect for bacteria vs. human cells at these wavelengths, and indeed
both 405-nm light [51] and 254-nm light [52] have been shown to
be mutagenic to mammalian cells at relevant fluences.
Based on the pre-clinical in-vitro studies reported here, 207-nm
light from a Kr-Br excimer lamp may have considerable potential
for safely reducing SSI rates. Such lamps could potentially be usedin an operating room environment without the need for protective
clothing for staff or patient. Of course in-vivo studies are anecessary confirmation and these are ongoing, as discussed below.
Should in-vivo studies confirm these in-vitro results, our expectation of the potential use of 207-nm light would be for continuous
low-fluence-rate exposure during a surgical procedure. The
rationale here is that current evidence suggests that the majorityof SSI result from bacteria alighting directly onto the surgicalwound from the air. Evidence for the dominance of an airborne
route comes from correlations between the density of airborne
bacteria and postoperative sepsis rates [53,54]; evidence for thesignificance of airborne bacteria alighting directly on the surgicalwound comes, for example, from studies of conventional UV
lamps specifically directed over the surgical site [8], and also
wound-directed filtered airflow studies [55].
Thus a continuous low-fluence-rate exposure of 207-nm UV
light onto the surgical wound area during the complete surgicalprocedure might be anticipated to kill bacteria as they alighted
onto the wound area. Such a continuous exposure, as also used in
the earlier studies of wound irradiation with conventional
germicidal lamps [8], would be designed to inactivate bacteria
before they penetrated into the interior of the wound. A second
advantage associated with targeting bacteria as they alight onto thewound area is in relation to biofilms [56]: as bacteria alight onto
the skin/wound, they are typically in individual planktonic form,
and thus amenable to killing by 207-nm light. This would preventthe subsequent formation of bacterial clusters (biofilms), which
would probably be refractory to 207-nm light (as they are to mostother therapies).
Several configurations for the 207-nm lamp in a surgical setting
might be possible. It is of course important that surgical staff do
not inadvertently block the UV light, so one arrangement might befor the excimer lamp to be incorporated into a standard overhead
surgical illumination system. A possible second UV light source, toensure a level of redundancy from inadvertent shielding, might beincorporation into a surgeon’s headlight illumination system, with
the UV light transmitted to the headlight via fiber optics [57].
Commercial 207-nm excimer lamps would be expected to be
inexpensive and long lived. In fact in a different wavelength range(172 nm) xenon excimer lamps [58] with rated lifetimes of 50,000
to 100,000 hrs, are already in commercial use [34], providing(using appropriate phosphors) both interior and exterior officelighting, for example on the Rotterdam KPN Telecom tower [59].
Though beyond the scope of the current work, in addition to
direct bactericidal applications in surgical environments, it might
be envisioned that 207-nm UV light could be used to treat, on a

2 Induction of Pre-mutagenic DNA Lesions in Human
Skin
Figure 5 shows the measured induced yields of CPD lesions(Figure 5a) and 6-4 photoproducts (6-4PP, Figure 5b) after
exposure of the 3-D skin tissue model to the broad UV spectrum
from a conventional germicidal UV lamp (peak 254 nm) or to 207nm light from the Kr-Br excimer lamp. As expected, thegermicidal lamp produced high yields of both pre-mutagenic skin
DNA lesions. However after 207-nm exposure, neither lesion
showed an induced yield which was significantly elevated above
zero, at any of the studied fluences.

Discussion
The results shown here are in accord with biophysicalexpectations based on the limited penetration of 207-nm UV
light: in terms of cell killing, 207-nm light does indeed kill MRSA
much more efficiently than it kills human cells. For example at afluence of 207-nm light that produce four logs of MRSA cell kill,
there is much less than one decade of cell kill in human cells (asdiscussed above, we chose four logs of bacterial kill for thiscomparison because studies of wound irradiation with conventional UV germicidal lamps suggest that this level of bacterial
reduction will produce significant decreases in SSI rates [8]). In
contrast with the results for 207-nm light, while conventional
germicidal lamps efficiently kill MRSA, conventional germicidal

Figure 5. Pre-mutagenic skin DNA lesion yields induced by aconventional germicidal lamp and by 207-nm UV light. Yields
are shown of pre-mutagenic DNA lesions in epidermal keratinocytes,
measured in a 3-D human skin tissue model, induced by conventional
broad-spectrum germicidal UV radiation (&) and by 207-nm UV light(¤). A: cyclobutane pyrimidine dimers (CPD); B: pyrimidine-pyrimidone 6-4 photoproducts (6-4PP). In both graphs zero-fluence control
measurements (,1%) have been subtracted from the data.
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continuous basis, any airborne environment where there is a highlikelihood of airborne-based pathogen transmission, such as TB or
pandemic influenza. In fact upper-room UV irradiation systems
have long been considered, based on conventional broad-spectrum
UV lamps [60], and have shown some promise [61,62]; they havenot, however, been widely used, in part because of safety concerns
relating to potential low-level broad-spectrum UV exposure[61,63,64].

sharing with conventional UV germicidal lamps [8] the majoradvantage of being equally effective for drug-resistant and drugsensitive bacteria.
Of course before 207-nm light approach were to be usedclinically in a surgical scenario, a number of issues remain to beaddressed: In vivo, studies of safety and efficacy in animal models of
SSI are ongoing, including the potential effects of UVC exposure
on wound healing - though earlier studies [66] have not shown
significant effect on wound healing.

Conclusions
Acknowledgments

There is clearly no single solution to the major problem of SSI[65]. Conventional germicidal UV lamps are effective but
represent a health risk both for patient and staff. Basic biophysics
considerations suggest, and our in-vitro data provide someconfirmation that, 207-nm UV light has a considerably improved
therapeutic ratio (bacterial killing vs. cell human killing) compared
to conventional germicidal UVC lamps [50], and also that 207-nm
UV light is unlikely to be mutagenic to humans. Based on these
considerations, 207-nm light has considerable promise to be safe
and inexpensive modality for SSI reduction, while potentially

We are very grateful to Lynn Shostack for her support and advice. This
work is dedicated to the memory of Paul Rice.

Author Contributions
Conceived and designed the experiments: GR-P FDL HMS SMH DJB.
Performed the experiments: MB AWB ST. Analyzed the data: MB GR-PAWB DJB. Contributed reagents/materials/analysis tools: AWB FDL
DJB. Wrote the paper: MB GR-P AWB DJB.

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