Abstract
Cryptococcus neoformans is the most frequent cause of fungal meningoencephalitis in immunocompromised patients. Because of its minimal side effects and ready penetration into the central nervous system, fluconazole (triazole), has been the most widely used for the treatment as well as for the long term maintenance therapy for cryptococcosis. As with other fungi, fluconazole resistance associated with mutation or overexpression of P-450 lanosterol 14a-demethylase (ERG11) or constitutive upregulation of multidrug transporters have been reported in C. neoformans. In addition, we have reported a unique pattern of resistance to triazoles termed heteroresistance in two strains of C. neoformans isolated from patients in 1998. Heteroresistant strains produce cultures with heterogeneous compositions in which most of the cells are susceptible but cells with highly resistant to fluconazole (MICs, ≥ 64 ug/ml) are recovered at a variable frequency. Highly resistant population can be obtained in homogeneity by subculturing resistant clones on media with drug but homogeneous population of sensitive culture can never be obtained from heteroresistant strains. Furthermore, the highly resistant culture derived from a resistant subpopulation is lost during repeated transfer on drug free media without loosing the basal subpopulation of resistant cells. In order to determine the intrinsic nature of heteroresistance, we screened over 100 Cryptococcus strains isolated at least 10 to 20 years prior to the birth of azole drugs. Surprisingly, considerable number of the strains isolated between 1960 to 1973 manifested heteroresistance to fluconazole indicating that heteroresistance in C. neoformans is intrinsic.
