5-Fluorocytosine resistance in Candida spp. and Torulopsis glabrata

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Abstract
Candida albicans, other Candida spp. and Torulopsis glabrata isolated from untreated patients were resistant to 5-fluorocytosine (5-FC) in 10 of 135 (75%) 14 of 43 (33%) and 3 of 50 (6%) cases respectively in that the minimal inhibitory concentration was 25·6 μg/ml or greater after 7 days incubation. Resistance was more common among yeasts isolated from patients treated with the drug. Resistant strains occurred more often when the dosage of 5-FC was low. Two types of resistance were found. One conferred insensitivity to 5-FC at the highest concentration tested (1 638 μg/ml). In the other, which was more common, the organisms were markedly affected by Iow concentrations of 5-FC, but after 7 days of incubation formed colonies in the presence of very high levels of the drug. 5-FC-resistant strains, probably induced by therapy, were isolated from treated patients for considerable periods after cessation of therapy. The main drawback to the use of the drug seems to be the occurrence of resistant strains so it should be used only when strictly indicated.