Mechanisms of fluconazole resistance in Candida albicans isolates from Japanese AIDS patients

Four Candida albicans isolates, TIMM 3163, TIMM 3164, TIMM 3165 and TIMM 3166, with reduced fluconazole susceptibility were obtained from three AIDS patients in Japan, and the mechanisms of their drug resistance were studied. All isolates showed lower levels of intracellular accumulation of fluconazole than ATCC 10231, a susceptible control strain of C. albicans. Increased amounts of CDR1 and CDR2 mRNA encoding putative ATP binding cassette (ABC) transporters were associated with the azole resistance of all TIMM isolates, apart from TIMM 3164. In addition, increased Cdr1p levels were immunodetected in the cell membrane fractions of all the TIMM strains except for TIMM 3164. Gene amplification was not responsible for CDR1 overexpression and there were no significant differences in the mRNA levels of CDR3 or CDR4 (ABC transporters) in the azole-susceptible and -resistant cells. CaMDR1 (a major facilitator superfamily) gene expression was not observed in any of the resistant isolates or the control strain. These results suggest that energy-dependent drug efflux associated with increased expression of CDR1 and CDR2 is involved in the fluconazole resistance mechanisms in two of the four isolates, TIMM 3165 and TIMM 3166. TIMM 3164 demonstrated energy-dependent drug efflux without overexpression of CDR1-4 or CaMDR1, indicating that some other pump may be operating. Despite showing low levels of drug efflux and overexpression of CDR1 and CDR2, efflux in TIMM 3163 was not energy dependent, suggesting that the expressed Cdr1p non-functional Cdr1p and that other resistance mechanisms may operate in this strain.