Ultrastructural changes in Paracoccidioides brasiliensis yeast cells attenuated by gamma irradiation.

Paracoccidioides brasiliensis is a thermally dimorphic fungus agent of paracoccidioidomycosis, a deep-seated systemic infection of humans with high prevalence in Latin America. Until now no vaccine has been reported. Ionizing radiation can be used to attenuate pathogens for vaccine development and we have successfully attenuated yeast cells of P. brasiliensis by gamma irradiation. The aim of the present study was to examine at ultrastructural level the effects of gamma irradiation attenuation on the morphology of P. brasiliensis yeast cells. Paracoccidioides brasiliensis (strain Pb-18) cultures were irradiated with a dose of 6.5 kGy. The irradiated cells were examined by scanning and also transmission electron microscopy. When examined 2 h after the irradiation by scanning electron microscopy, the 6.5 kGy irradiated cells presented deep folds or were collapsed. These lesions were reversible since when examined 48 h after irradiation the yeast had recovered the usual morphology. The transmission electron microscopy showed that the irradiated cells plasma membrane and cell wall were intact and preserved. Remarkable changes were found in the nucleus that was frequently in a very electrondense form. An extensive DNA fragmentation was produced by the gamma irradiation treatment.