Abstract: In order to investigate some requisite conditions for the formation of mycelial phase of Pityrosporum orbiculare (P.orb) and understand indirectly the pathogenesis of tinea versicolor, we used a semisolid medium containing cholesterol 1.0g/dl and agar 0.8g/dl, and obtained the mycelial phase of P.orb.Then we studied the relationship between the formation of the mycelial phase and the concentration of cholesterol or agar in the medium.The results showed that when P.orb were cultured on the medium, the hyphae began to grow on the 4-6th day, increased gradually, and peaked on the 10-12th day.The hyphae were mostly blastohyphae, 8-16μm in length and in the shape of racket,and distributed in clustes.It was found that 0.25g/dl was the minimum concentration of cholesterol to induce the mycelial phase in vitro.Within the range of 0.25-1.0g/dl the higher the concentration was, the more the hyphae were.When the concentration exceeded 1.0g/dl, the hyphae did not increase markedly.Relative high moisture (0.8g/dl of agar in the medium) was suitable for the production of the hyphae.The results suggest that the occurrence and recurrence of tinea versicolor in summer probably relate to the change of choleserol concentration on the surface of the skin after the evaporation of sweat.

Key words: Malassezia, Tinea versicolor