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Two nystatin-resistant mutants of Candida were isolated in vivo and identified as C. albicans and C. krusei. Analysis of the sterol composition of these cells indicated a total absence of ergosterol and an increased level of a possible precursor. Successive cultures of the resistant strains in a medium supplemented with 10 microgram ergosterol ml-1 induced in 5d a sensitivity to nystatin identical to that of the wild-type strain. These ergosterol-supplemented strains had an ergosterol content similar to that of the wild-type strain cultured in the absence of ergosterol. Resistance to nystatin was recovered after 5d subculture in medium without ergosterol.

SUMMABYEthidium bromide and acriflavin-resistant mutants of petite-negative yeast Kluyveromyces lactis were prepared. One kind of nuclear mutation (EB E1 ) gave resistance to ethidium bromide and correlated sensitivity towards acriflavin. Another nuclear mutation (EB R2 ) did not affect 'natural' resistance of this yeast towards 15 /IM. acriflavin. Both nuclear mutations mapped at different loci, suggesting lack of linkage. Cytoplasmic mutants resistant to these two drugs were unstable when grown in complete media with dextrose, reverting to a wild-type resistance genotype. When grown in glycerol-containing media these mutants maintained their cytoplasmic drug-resistance conferring factors.