Effect of elevated temperatures and low levels of trace metals on the growth and phenotypic development of Candida albicans.

A combination of elevated temperatures (within the human febrile range) and trace metal chelation were investigated for their effects on the inhibition of growth and phenotypic development of the dimorphic yeast Candida albicans (strain 3153A). The ability of specific cations to relieve the phenotypic inhibition that occurred also was tested. Elevated temperatures alone (to 41 degrees C) only delayed the timing of the phenotypic development. When compared to the results obtained at 37 degrees C, the recombination of elevated temperature and addition of the trace metal chelator, 1,10-phenanthroline, did not further suppress phenotypic development, but the combination did decrease the viability of C. albicans. When 24 to 48 h stationary phase singlet cells were released into a medium containing 100 microM 1,10-phenanthroline (pH 6.5), supplemental iron (200 microM) alleviated the suppression of mycelium formation at 41 degrees C, whereas under conditions favoring bud formation (pH 4.5), both iron and zinc circumvented suppression and promoted budding. Through studies on the interaction of temperature stress and trace metal availability our data revealed the requirement for iron mycelium formation whereas both iron and zinc may be needed for bud formation.

Inhibition of the differentiation of Candida albicans by the chelator 1,10-phenanthroline.

Several chelators were examined for their ability to prevent the synchronous release of 24- to 48-hour stationary phase singlet cells of the dimorphic yeast Candida albicans into either the mycelial or the budding phenotypes (in a defined liquid medium at 37 degrees C; at pH 6.5 or pH 4.5, respectively). The only chelator that was found to inhibit mycelium formation completely and to restrict bud formation to about 10% was 1,10-phenanthroline at minimal concentrations of 50 microM and 230 microM, respectively. The inhibition of both phenotypes could be reversed completely by the addition of 200 microM of ZnSO4. The synchrony of recovery from inhibition by the addition of zinc paralleled that of the controls for both phenotypes, and the final number of mycelia or buds as a percentage of the control was the same (100%). These findings support the hypothesis that the lag period between the release from stationary phase and the onset of development for Candida represents the time of acquisition of a minimum threshold amount of a cation, such as zinc. The involvement of zinc in phenotypic development is discussed, suggesting that while zinc is involved in the initiation of development, it may not determine the phenotype of Candida albicans.

Effect of temperature on siderophore production by Candida albicans.

The purpose of this study was to examine the effect of elevated temperature on growth and siderophore production by Candida albicans. The results showed that an increase in incubation temperature from 37 degrees C to 41 degrees C produced a marked decrease in both the rate and quantity of siderophore production. Elevated temperature was unable to suppress growth of C. albicans in either a control culture medium or a deferrated culture medium. A significant suppression of growth compared to the controls was observed in the deferrated media at both 37 degrees C and 41 degrees C. However with time, the growth of cells in the deferrated media showed partial recovery which was followed by an increase in siderophore production. Thus, elevation of temperature to suppress growth and siderophore production by C. albicans appears to be an ineffective host defense mechanism.

Siderophore production by the pathogenic yeast, Candida albicans.

Biochemical assays were used to determine that some strains of Candida albicans were capable of simultaneous secretion of both the hydroxamate and phenolate-type siderophores when grown in a deferrated medium at 37 degrees C. All isolants of C. albicans released hydroxamate-type siderophores into the culture medium; whereas, approximately 40% of the strains simultaneously secreted phenolate-type siderophores. The presence of phenolate and hydroxamate-type siderophores in the culture medium was further confirmed by assaying the culture media with type specific siderophore-dependent bacterial auxotrophs. This is the first report showing production of both classes of siderophores by a pathogenic yeast.

Zinc and regulation of growth and phenotype in the infectious yeast Candida albicans.

When Candida albicans is grown at 25 degrees C in suspension in defined medium, cells accumulate at stationary phase as singlets in G1 of the deoxyribonucleic acid replication cycle and acquire the capacity to form mycelia. When cells were removed from a stationary-phase culture and a low concentration of fresh cells was inoculated into the cell-free, stationary-phase medium, the fresh cells grew to approximately the same cell density as the original culture. We demonstrated that in the accompanying decrease in pH, nor due to a depletion of O2, an accumulation of CO2, a physical crowding effect, or accumulation of the putative autoinhibitors tryptophol and 2-phenylethyl alcohol. Rather, cells stop multiplying at stationary phase due to the depletion of zinc from the culture medium. The manipulation of cultures with glassware to remove stationary-phase cells and to add fresh cells led to the addition of zinc to the medium and hence a new round of culture growth. The same manipulations with plasticware did not result in zinc supplementation and hence in now new round of culture growth. When cells enter stationary phase in excess zinc, they do not accumulate as singlets; rather, they accumulate as budded cells. When these cells were induced to form mycelia, they did so in half the time it took zinc-starved cells. The usefulness of employing zinc starvation as a method for obtaining a uniform stationary-phase phenotype and for synchronizing induced mycelium or bud formation is discussed.

Effects of low concentrations of zinc on the growth and dimorphism of Candida albicans: evidence for zinc-resistant and -sensitive pathways for mycelium formation.

In this analysis we have examined in detail the effects of low concentrations of zinc on the growth and dimorphism of Candida albicans. Evidence is presented that micromolar concentrations of zinc added to growth cultures grown at 25 degrees C (i) cause a twofold increase in the final concentration of spheres at sationary phase, (ii) result in an asynchronous block in the budding cycle at stationary phase, (iii) completely suppress mycelium formation in two independently isolated human strains which produce low but significant levels of mycelia at stationary phase, and (iv) completely suppress mycelium formation in cultures of mutant M10, in which over 60% of the cells form mycelia at stationary phase. In contrast, micromolar concentrations of zinc do not inhibit mycelium formation induced by releasing cells from stationary-phase cultures into fresh medium at 37 degrees C. In addition, if zinc is present in the growth medium of the initial culture at 25 degrees C, the average time of subsequent mycelium formation after release into fresh medium at 37 degrees C is halved. It is demonstrated that the above effects are specific to zinc. The possibility of alterante pathways for mycelium formation is suggested, and the medical implications of this possibility are discussed.