Disinfection and healing effects of 222-nm UVC light on methicillin-resistant Staphylococcus aureus infection in mouse wounds

Abstract

UVC radiation is known to be highly germicidal. However, exposure to 254-nm-UVC light causes DNA lesions such as cyclobutane pyrimidine dimers (CPD) in human cells, and can induce skin cancer after long-term repeated exposures. It has been reported that short wavelength UVC is absorbed by proteins in the membrane and cytosol, and fails to reach the nucleus of human cells. Hence, irradiation with 222-nm UVC might be an optimum combination of effective disinfection and biological safety to human cells. In this study, the biological effectiveness of 222-nm UVC was investigated using a mouse model of a skin wound infected with methicillin-resistant Staphylococcus aureus (MRSA). Irradiation with 222-nm UVC significantly reduced bacterial numbers on the skin surface compared with non-irradiated skin. Bacterial counts in wounds evaluated on days 3, 5, 8 and 12 after irradiation demonstrated that the bactericidal effect of 222-nm UVC was equal to or more effective than 254-nm UVC. Histological analysis revealed that migration of keratinocytes which is essential for the wound healing process was impaired in wounds irradiated with 254-nm UVC, but was unaffected in 222-nm UVC irradiated wounds. No CPD-expressing cells were detected in either epidermis or dermis of wounds irradiated with 222-nm UVC, whereas CPD-expressing cells were found in both epidermis and dermis irradiation with 254-nm UVC. These results suggest that 222-nm UVC light may be a safe and effective way to reduce the rate of surgical site and other wound infections.

Keywords: 222-nm UVC; CPD; Dermis; Methicillin-resistant Staphylococcus aureus; Wound infection.

Fig. 1

Measured spectra emitted from the Kr-Cl excimer lamp equipped with a band- pass filter.

Fig. 2

Bactericidal effect of 222-nm UVC irradiation on S. aureus inoculated onto mouse dorsal skin. S. aureus suspension was inoculated onto dorsal skin of mice, and air-dried for 1 h. The skin was irradiated with 222-nm UVC light at 75, 150 and 450 mJ/cm². Bacterial counts in the skin were enumerated as described in the materials and methods. Data are expressed as the means ± standard deviation of a group of 5 mice. * P < 0.05.

Fig. 3

Bactericidal effect of 222-nm UVC irradiation on S. aureus inoculated onto skin wounds. Full-thickness wounds were made on mouse dorsal skin, and S. aureus suspension was spotted onto the wounds. The wounds were irradiated with 222-nm and 254-nm UVC light at 75 (A), 150 (B), 750 (C) and 1500 mJ/cm² (D) at 1 h after inoculation. Bacterial counts were enumerated immediately and 1 day after irradiation. Data are expressed as the means ± standard deviation of a group of 3 to 4 mice. ** P < 0.01, * P < 0.05, ND; not detected.

Fig. 4

Time courses for bacterial count in S. aureus-infected wounds after 222-nm and 254-nm UVC irradiation. S. aureus suspension was inoculated onto the skin wounds of mice, and the wounds were irradiated with 75 mJ/cm² of 222-nm (A) or 150 mJ/cm² of 222-nm and 254-nm UVC light (B) at 1 h after inoculation. Skin samples were taken on days 3, 5, 8 and 12 after UVC irradiation, and bacterial counts in the wounds were enumerated. Open square: non-irradiated wound, closed square: 254-nm UVC irradiated wound and closed circle: 222-nm UVC irradiated wound. Data are expressed as the means ± standard deviation of a group of 3 to 4 mice. * P < 0.05.

Fig. 5

Gross appearances of S. aureus-infected skin wounds after UVC irradiation. S. aureus suspension was inoculated onto the skin wounds of mice, and the wounds were irradiated with 150 mJ/cm² of 222-nm and 254-nm UVC light at 1 h after inoculation. Wounds were observed immediately and on days 3, 5, 8 and 12 after UVC irradiation (A). The skin wounds were entirely healed until day 18 after UVC irradiation (B). Scales bars represent 5 mm.

Fig. 6

Histology of S. aureus-infected wounds after UVC irradiation. Mouse skin wounds were infected with S. aureus and then irradiated with 150 mJ/cm² of 222-nm and 254-nm UVC light at 1 h after inoculation. Wound tissue samples were taken on day 5 (A–C) and day 8 (D–F) after irradiation. Tissue sections were stained with hematoxylin and eosin (A, D). Arrows indicate migration of epidermal cells. Immunohistochemistry was performed to detect neutrophils (B, E). Arrows indicate neutrophil infiltration. S. aureus cells were detected by immunohistochemistry (C, F). Arrows indicate S. aureus aggregates.

Fig. 7

Effect of 222-nm UVC exposure on DNA damage in mouse dorsal skin. Mice dorsal skins were irradiated with 222-nm or 254-nm UVC at 150 mJ/cm² and excised immediately (A) and on day 1 after irradiation (B). CPD-expressing cells were detected by immunohistochemical staining (red-brown). An arrow indicates inflammatory cell infiltration. (C) CPD-positive cells were quantified by counting the cells in 10 random high-power (× 400) fields of each section.

Fig. 8

Effect of 222-nm UVC exposure on DNA damage in S. aureus infected wound tissue. Mouse skin wounds were made and infected with S. aureus. The wounds were irradiated with 150 mJ/cm² of 222-nm and 254-nm UVC light at 1 h after infection. Wound tissue samples were taken immediately (A) and on day 1 after irradiation (B). CPD-retaining cells were detected by immunohistochemical staining (red-brown). Black arrows indicate wound surface. Red arrow indicates epidermis.