Usual concentrations of antimycin A, rotenone and EDTA, individually or in combination, reduced aerobic growth rate and cell yield of Candida albicans to about half its normal level and to about the levels of previously-described acetate-negative, cytochrome-complete and aa3-deficient variants which were little affected by the inhibitors. Anaerobic conditions (not affected by antimycin A) reduced growth rate and cell yield of all cultures-including that of a nonrespiring aa3, b-deficient mutant-to low, equal levels. Antimycin A but not rotenone prevented growth of the normal strain on ethanol medium. Cyanide and antimycin A blocked most of the respiration of the normal strain and cytochrome-complete variant, but did not affect that of the cytochrome aa3-deficient mutant. Rotenone and EDTA did not affect respiration of any of the cultures. SHAM blocked cyanide-and antimycin A-insensitive respiration and prolonged the lag phases of the three respiring cultures, especially in the presence of antimycin A, but alone increased oxygen-uptake rate of the cytochrome-complete cultures while curtailing that of the cytochrome aa3-deficient mutant. Resting cells, especially wild-type, grown in medium containing antimycin A exhibited lowered oxygen-uptake rate, which was increased upon the addition of cyanide or antimycin A. Antimycin A stimulated, but cyanide inhibited, respiration of cytochrome-complete cultures grown in the presence of rotenone but did not affect that of the cytochrome aa3-deficient mutant. SHAM inhibited respiration of all antimycin A- or rotenone-grown cultures. The high rate of respiration of C. albicans in the presence of inhibitors for three sites of electron transport in the conventional oxidative pathway, the inhibition of this respiration by SHAM and its loss by the absence of cytochrome b, indicate an alternate oxidative pathway in this organism which crosses the conventional one at cytochrome b.

A number of acriflavine-induced mutants of Candida albicans, characterized by their inability to grow on acetate as a source of energy, were screened for their cytochrome absorption spectra. Three mutants with different spectra, along with their parent, were selected for comparative studies of their growth, respiratory activities and cellular structure. The spectrum of one of the mutants was the same as that of the wild-type, but the growth rate and yield of cells on glucose medium were only about 60% of the wild-type's; those of a second mutant deficient in cytochromes aa3 were 50%, and those of a third mutant deficient in cytochromes aa3 and b were less than 5% of those of the wild-type. The cytochrome-complete mutant and the wild-type showed respiratory activity both on glucose and ethanol well above the endogenous, the cytochrome aa3-deficient mutant showed only endogenous respiration, and the cytochrome aa3, b-deficient mutant no respiration at all. Electron microscopy of the wild-type cells revealed discrete, regular ovoidal, cristate mitochondria spaced near the periphery of the protoplasm; the cytochrome-complete mutant showed an abundance of large, cristate, but morphologically irregular mitochondria; the cytochrome aa3-deficient mutant had fewer but still large, cristate, somewhat irregular mitochondria; and the cytochrome aa3, b-deficient mutant only a few simple vesicles without discernible cristae.