Ultraviolet C inactivation of dermatophytes: implications for treatment of onychomycosis

Abstract

Background: Onychomycosis responds to systemic antifungals and sometimes to topical lacquers, but alternative treatments are desirable. Topical application of germicidal ultraviolet (UV) C radiation may be an acceptable and effective therapy for infected nails.

Objectives: To test the ability of UVC to inactivate dermatophyte suspensions in vitro and to sterilize a novel ex vivo model of nail infection.

Methods: Trichophyton rubrum, T. mentagrophytes, Epidermophyton floccosum and Microsporum canis suspensions were irradiated with UVC (254 nm) at a radiant exposure of 120 mJ cm(-2) and surviving colony-forming units quantified. T. rubrum infecting porcine hoof slices and human toenail clippings was irradiated with UVC at radiant exposures of 36-864 J cm(-2).

Results: In vitro studies showed that 3-5 logs of cell inactivation in dermatophyte suspensions were produced with 120 mJ cm(-2) UVC irradiation. Depending on factors such as the thickness and infectious burden of the ex vivo cultures, the radiant exposure of UVC needed for complete sterilization was usually in the order of tens to hundreds of J cm(-2). Resistance of T. rubrum to UVC irradiation did not increase after five cycles of subtotal inactivation in vitro.

Conclusions: UVC irradiation may be a less invasive treatment option for onychomycosis, when the appropriate consideration is given to safety.

Fig 1

Dose responses to ultraviolet (UV) C irradiation of different dermatophyte suspensions under the same optical density (OD_{570 nm} ≈ 6·5). The data are representative experiments performed in triplicate and are displayed as mean ± SD. T. rubrum, Trichophyton rubrum; T. mentagrophytes, Trichophyton mentagrophytes; M. canis, Microsporum canis; E. floccosum, Epidermophyton floccosum.

Fig 2

Dose responses of Trichophyton rubrum in vitro to five consecutive subtotal ultraviolet (UV) C inactivations. The data are representative experiments performed in triplicate and are displayed as mean ± SD.

Fig 3

Periodic acid–Schiff-stained histology of (a) a 4-week porcine hoof culture infected with Trichophyton rubrum; (b) a noninfected porcine hoof fragment. Bars = 200 μm.

Fig 4

(a) Consecutive microtome-sliced 50-μm thick sections on Sabouraud dextrose agar (SDA) from an ex vivo porcine hoof culture of Trichophyton rubrum irradiated with an ultraviolet C exposure of 576 J cm⁻² (8 h irradiation at an irradiance of 20 mW cm⁻²); (b) Consecutive 50-μm thick sections from a nonirradiated porcine hoof culture. Numbers indicate the depths (μm) of the sections from the hoof surface. Day 14 culture on SDA.

Fig 5

Time courses of Trichophyton rubrum colony outgrowth size (diameter) on Sabouraud dextrose agar from a porcine hoof culture irradiated with an ultraviolet (UV) C exposure of 576 J cm⁻² (8 h irradiation at an irradiance of 20 mW cm⁻²) and a nonirradiated control, respectively.

Fig 6

Time delays of Trichophyton rubrum colony outgrowth on Sabouraud dextrose agar from microtome-sliced 50-μm thick sections at different depths of an ultraviolet (UV) C-irradiated porcine hoof culture. UVC exposure: 864 J cm⁻².