The effect of 5-fluorocytosine on the germination of Aspergillus fumigatus and on subsequent nuclear division.

5-Fluorocytosine (5FC) inhibits the germination of the conidia of A. fumigatus. This is different from the lack of effect of 5FC on hyphal formation from blastospores of C. albicans because, unlike C. albicans, A. fumigatus requires DNA synthesis for germination, and this is inhibited by 5FC. A modified fluorescent Feulgen method has been developed to stain fungal nuclei in the presence and absence of 5FC. This method has shown that hyphae treated with 5FC have fewer nuclei than untreated hyphae. During hyphal growth, DNA synthesis and hyphal elongation in A. fumigatus are equally inhibited by 5FC. This is different from the differential effect of 5FC on C. albicans when hyphal elongation continues during an inhibition of DNA synthesis. The equal inhibition in A. fumigatus is clearly not unbalanced growth and so the action of 5FC on A. fumigatus is merely fungistatic.