We tested the hypothesis that adaptation of Candida albicans to chronic oxidative stress inhibits the formation of hyphae and reduces pathogenicity. Candida albicans cells were exposed to increasing concentrations of t-butylhydroperoxide (tBOOH), a lipid peroxidation-accelerating agent, and mutants with heritable tBOOH tolerance were isolated. Hypha formation by the mutants was negligible on Spider agar, indicating that the development of oxidative stress tolerance prevented Candida cells from undergoing dimorphic switches. One of the mutants, C. albicans AF06, was five times less pathogenic in mice than its parental strain, due to its reduced germ tube-, pseudohypha- and hypha-forming capability, and decreased phospholipase secretion. An increased oxidative stress tolerance may therefore be disadvantageous when this pathogen leaves blood vessels and invades deep organs. The AF06 mutant was characterized by high intracellular concentrations of endogenous oxidants, reduced monounsaturated and polyunsaturated fatty acid contents, the continuous induction of the antioxidative defense system, decreased cytochrome c-dependent respiration, and increased alternative respiration. The mutation did not influence growth rate, cell size, cell surface, cellular ultrastructures, including mitochondria, or recognition by human polymorphonuclear leukocytes. The selection of oxidative stress-tolerant respiratory Candida mutants may also occur in vivo, when reduced respiration helps the fungus to cope with antimycotic agents.

The effect of antioxidant ascorbic acid (vitamin C) pretreatment on chromium(VI)-induced damage was investigated using the yeast Saccharomyces cerevisiae as a model organism. The objective of this study was to pretreat yeast cells with the antioxidant ascorbic acid in an effort to increase cell tolerance against reactive chromium intermediates and reactive oxygen species formed during chromium(VI) reduction. Intracellular oxidation was estimated using the fluorescence indicators dihidro-2,7-dichlorofluorescein, dihydroethidium and dihydrorhodamine 123. The role of ascorbic acid pretreatment on chromium(VI) toxicity was determined by measuring mitotic gene conversion, reverse mutations, 8-OHdG, hydroxyl radical, superoxide anion and chromium(V) formation. The chromium content in the biomass was determined by flame atomic absorption spectrometry. In the absence of chromium, ascorbic acid effectively protected the cells against endogenous reactive oxygen species formed during normal cellular metabolism. In vitro measurements employing EPR and the results of supercoiled DNA cleavage revealed that the pro-oxidative action of ascorbic acid during Cr(VI) reduction was concentration-dependent and that harmful hydroxyl radical and Cr(V) had formed following Cr(VI) reduction. However, the in vivo results highlighted the important role of increased cytosol reduction capacity related to modification of Cr(V) formation, increased chromium accumulation, better scavenging ability of superoxide anions and hydrogen peroxide, and consequently decreased cytotoxicity and genotoxicity in ascorbic acid pretreated cells. Ascorbic acid influenced Cr(VI) toxicity both as a reducing agent, by decreasing Cr(V) persistence, and as an antioxidant, by decreasing intracellular superoxide anion and hydrogen peroxide formation and by quenching free radicals formed during Cr(VI) to Cr(III) reduction. Increased 8-OHdG and decreased reduced glutathione in ascorbic acid-treated cells might induce an endogenous antioxidant defense system and thus increase cell tolerance against subsequent Cr-induced stress.

The yeast Candida intermedia, as a model organism, was used to examine the links between the metal ions exposure, reactive oxygen species generation and oxidative stress response. To estimate intracellular peroxide and superoxide levels, the fluorescence indicators dihydrorhodamine 123 and dihydroethidium were used, respectively. Antioxidant defence systems were investigated by measuring the activity of catalase, glutathione peroxidase and superoxide dismutase and the content of reduced glutathione. Altered superoxide, peroxide, glutathione levels, and the catalase activity were perceived after the treatment with copper. In the samples treated with selenium and zinc the altered peroxide and superoxide levels, and the glutathione peroxidase activity were perceived. The results indicate that the tolerance of the yeast C. intermedia to different metal ions was correlated with the reactive oxygen species generation in the cells and with the efficiency of antioxidative defence systems.

Reactive oxygen species (ROS) are not only generated in conditions of cellular stress but are also constitutively produced in most cell types by specific metabolic processes. This research focused on a potential antioxidant Trolox (model compound for alpha-tocopherol), with the aim to establish exact mechanisms of Trolox intracellular oxidation prevention on model organism Saccharomyces cerevisiae. Measuring intracellular oxidation of Trolox-treated yeast cells revealed that Trolox decreased intracellular oxidation during normal metabolism. Trolox treatment decreased cyto- and geno-toxicity of treated yeast cells in MES buffer, lowered intracellular oxidation, decreased intracellular peroxides formation, and increased H(2)O(2) degradation and superoxide quenching yeast extract ability. This study suggests that Trolox treatment provides prevention against intracellular ROS formation. Trolox application as therapeutic agent against intracellular ROS formation would be worth considering. Additionally, results indicate that yeasts are good model organisms for studying intracellular oxidation and oxidative stress. The obtained results on yeast cells might be useful to direct further human-related search for the Trolox evaluation as a human supplement used for protecting cells against intracellular free radical formation.

Although exposure of Candida albicans cells to high-dose (4 mM) methylprednisolone stimulated microbial growth, germination rate in serum and phospholipase release, it also promoted the recognition of C. albicans cells by polymorphonuclear leukocytes. Pretreatment of C. albicans cells with methylprednisolone did not result in any increase in the pathogenicity of the fungus in intraperitoneal and intravenous mouse assays. Therefore, the virulence of C. albicans is unlikely to increase in patients treated with comparably high-dose methylprednisolone on skin and mucosal membranes. Methylprednisolone treatments also increased the production of conjugated dienes and thiobarbituric acid-reactive substances, and the menadione sensitivity of C. albicans cells, which can be explained by a significant decrease in the specific activities of several antioxidant enzymes. The combination of methylprednisolone with oxidants, e.g. in topical applications, may be of clinical importance when the predisposition to candidiasis is high. Methylprednisolone treatments negatively affected membrane fluidity and decreased the antifungal effects of both the polyene antibiotic nystatin and the ergosterol biosynthesis inhibitor lovastatin, and also enhanced the deleterious effects of the polyene antimycotic amphotericin B on C. albicans cells. These corticosteroid-polyene drug interactions should be considered in the treatment of C. albicans infections in patients with prolonged topical application of corticosteroids.