Species Specific Susceptibility of Malassezia against Antifungal Agents in vitro

Yeasts of the genus Malassezia are part of the normal flora of human skin. Under appropriate conditions they may cause skin infections such as pityriasis versicolor, Malassezia folliculitis, se-borrhoeic dermatitis, and dandruff. It has been reported that Malassezia may be a trigger factor for atopic dermatitis. The in vitro antifungal susceptibility testing of Malassezia still remains a problem. There is no recommended method of antifungal susceptibility testing designed for lipophilic genus Malassezia. 81 strains of Malassezia were examined for their in vitro susceptibility to antifungal substances (17 were reference or pre-identified strains, 64 were obtained from clinical specimens). Identification of the clinical strains based on biochemical features and Fourier-Transform-Infrared-Spectroscopy. All strains were maintained 7 days on modified Dixon agar. In vitro susceptibility testing was performed on microtitration plates with laid out of 6 dehydrated antifungals (ketoconazole, voriconazole, itraconazole, fluconazole, amphotericin B, and 5-flucytosin) in 12 different concentrations. The yeast cell inoculum was adjusted to 5" 103 cfu (colony forming unit) suspended in modified Dixon without peptone. All Malassezia strains were very susceptible to the azole drugs. Especially, voriconazole (MIC - minimum inhibitory concentration -0.007 - 1 mg/ml), itraconazole (MIC 0.007 - 0.05 mg/ml), and ketoconazole (MIC 0.03 - 0.5 mg/ml) were the most effective agents. MIC values among Malassezia sympodialis, Malassezia globosa and Malassezia obtusa were comparable, whereas MIC values for M. furfur were significantly higher (p < 0.05). All values ranged among those accepted according the NCCLS standards. Amphotericin B was also effective against Malassezia in vitro (MIC 0.03125 - 8 mg/ml). 5-Flucytosin was unlikely to inhibit the growth of Malassezia. In some cases it could prevent the growth, however high concentrations were needed. There were variations in susceptibility of different Malassezia species to azole antifungal substances. Malassezia furfur was found to be the less susceptible species when compared to Malassezia obtusa, Malassezia globosa and Malassezia sympodialis.