UV SUSCEPTIBILITY AND NEGATIVE PHOTOTROPISM OF DERMATOPHYTES

High doses of ultraviolet radiation (UVR) have well-known inhibitory e ffects upon dermatophytes. In the present study, the effect of repetit ive low doses of UVR on mycelial growth of dermatophytes was tested. P ellets of Trichophyton rubrum, T. mentagrophytes and Microsporum canis ; were placed between two thin layers of Sabouraud glucose agar. Obver se, reverse or both sides of these 'sandwich' agars were irradiated fo r 10 days twice daily with 0.13 or 0.17 J cm(-2) UVB. To simulate micr oaerophilic conditions, one or both agar sides were covered by transpa rent airtight plastic lids. In addition, T. rubrum was also grown as u sual on plates of Sabouraud glucose agar without any covering, and irr adiated on its obverse side twice daily with UVA (13.5 J cm(-2)), UVB (0.17, 0.34 or 0.69 J cm(-2)) or infrared light, or once only with 3.8 -15.1 J cm(-2) UVB. As a result, thallus diameters of all strains were found to be reduced by repetitive UVB irradiation. Furthermore, all s trains reacted with a negative phototropism to repetitive UVB irradiat ion under both aerobic and microaerophilic growth conditions. T. rubru m was unaffected by infrared irradiation, responded with an increased pigmentation to UVA (13.5 J cm(-2) twice daily) and was inhibited by a single dose of 15.1 J cm(-2) UVB. Negative phototropism of dermatophy tes is a new observation. It may be biologically relevant as a mechani sm to evade harmful doses of UVR.