Paracoccidioides brasiliensis: virulence and an attempt to induce the dimorphic process with fetal calf serum.

The virulence of Paracoccidioides brasiliensis strains and the induction of the dimorphic process of the fungus were investigated. The strains were kept under mineral oil for different periods of time, except for one that was successively subcultured on agar slants. Four strains with transitional morphology at room temperature were avirulent to experimental animals. Two strains with typical morphology at 36 degrees C, one of them preserved under mineral oil for 16 years and the other maintained by successive subcultures for 10 years, were virulent to mice. These strains caused macro- and microscopic lesions in various organs, with enlargement of the spleen and heart. An attempt to induce the dimorphic process with fetal calf serum (FCS) in P.brasiliensis strains with transitional morphology failed. FCS was probably unable to stimulate the synthesis of Y cell wall alpha-1,3-glucan. The results demonstrate that keeping strains under mineral oil for a long period of time may have altered the enzymatic activities of the proteinases and other virulence factors that participate in the transition process and in pathogenesis.