Comparative morphological and biological studies on the itraconazole- and ketoconazole-resistant mutants of Cryptococcus neoformans

Studies were carried out on the resistance in vitro of Cryptococcus neoformans to the oral antifungal drugs itraconazole and ketoconazole. None of the six sensitive strains tested developed resistance to itraconazole or ketoconazole by serial transfer on Sabouraud's glucose agar plates containing increasing concentrations of either drug. One mutant resistant to itraconazole, and one mutant resistant to ketoconazole, were isolated from the progenies of yeast cells surviving after treatment with a mutagenic substance, N-methyl-N'-nitro-N-nitrosoguanidine. These mutants were capable of growing in the presence of high concentrations of the drugs to which they were resistant. The itraconazole- and ketoconazole-resistant mutants obtained by mutagenesis were compared morphologically and biologically. The itraconazole-resistant mutant was characterized by the formation of very rough colonies which varied in size and shape, production of a large number of cell clusters, complete loss of capsule formation, and major degenerative changes in the cells, while in the ketoconazole-resistant mutant these changes were less pronounced and no cell clusters were formed. The acquisition of resistance was more stable in the itraconazole-resistant mutant than in the ketoconazole-resistant mutant. Both mutants showed partial cross-resistance and complete loss of virulence for mice.