A Saccharomyces cerevisiae mutant with increased virulence

Abstract

Saccharomyces cerevisiae, bakers' yeast, is not a pathogen in healthy individuals, but is increasingly isolated from immunocompromised patients. The more frequent isolation of S. cerevisiae clinically raises a number of questions concerning the origin, survival, and virulence of this organism in human hosts. Here we compare the virulence of a human isolate, a strain isolated from decaying fruit, and a common laboratory strain in a mouse infection model. We find that the plant isolate is lethal in mice, whereas the laboratory strain is avirulent. A knockout of the SSD1 gene, which alters the composition and cell wall architecture of the yeast cell surface, causes both the clinical and plant isolates to be more virulent in the mouse model of infection. The hypervirulent ssd1⌬͞ ssd1⌬ yeast strain is a more potent elicitor of proinflammatory cytokines from macrophages in vitro. Our data suggest that the increased virulence of the mutant strains is a consequence of unique surface characteristics that overstimulate the proinflammatory response.B akers' yeast, Saccharomyces cerevisiae, is not a pathogen in healthy individuals, but is increasingly isolated from immunocompromised patients (1). This organism, used commercially in the preparation of food and beverages, is usually found in the environment associated with rotting fruit. The use of live Saccharomyces in the treatment of diarrhea in Europe has been linked to yeast sepsis, making it clinically relevant (1, 2). In addition to its use as an oral probiotic, live S. cerevisiae shows promise as a vaccine delivery vector (3), and the predominant yeast cell wall molecule, ␤-glucan, is a potent immunostimulatory molecule with potential clinical use (4). The possible infectious properties and clinical utility of S. cerevisiae highlight the importance of understanding the yeast-immune system interaction.Some clinical isolates of S. cerevisiae proliferate and cause death in DBA͞2 mice (5, 6), but not other standard laboratory mouse strains. Clinical yeast isolates are able to grow at higher temperatures (41°C) than laboratory strains, and this characteristic has been correlated with their survival in mice (7). Recent work has also shown that metabolic genes required for the virulence of more traditional human pathogens such as Cryptococcus neoformans and Candida albicans are also required for survival of clinical isolates in mice (8). These experiments have established that the mouse models of S. cerevisiae virulence are relevant to a broad range of fungal pathogens and are useful for identification of new determinants of fungal virulence.We have genetically characterized a number of Saccharomyces strains recently isolated from humans and found that in the CISC44 (CISC, clinical isolate of Saccharomyces cerevisiae) strain the SSD1 gene is heterozygous and required for hightemperature growth (Htg). In wild strains of S. cerevisiae isolated either from humans or plants, we show that knockout of the SSD1 gene, which alters the composition and cell wall architecture of the ye...

Application of SNPs for assessing biodiversity and phylogeny among yeast strains

“…These included 14 wine strains, 10 of which were isolated by RK Mortimer from one winery in California (Polsinelli et al, 1996) and four other strains of various types (Sauternes, Champagne, All Purpose and Hock). In addition, the analysis included two American commercial baking yeast: Red Star and Fleishman's, one beer strain, Danstar Windsor, one diploid strain obtained from South Africa, F2007, and two strains, CISC30 and CISC44, which were isolated from immunosuppressed patients who had systemic fungal infections (Wheeler et al, 2003). In addition, sequence information from three different Saccharomyces non-cerevisiae species (S. paradoxus, S. mikatae, S. bayanus), supplied by Kellis et al (2003), provided the genotypes for use as an outgroup.…”

Budding YeastSSD1-VRegulates Transcript Levels of Many Longevity Genes and Extends Chronological Life Span in Purified Quiescent Cells

“…One possible explanation is that SSD1-V, which encodes the full-length "wildtype" protein, reduces the virulence of Saccharomyces cerevisiae in one mouse model (Wheeler et al, 2003), but SSD1-V is critical in Candida (Gank et al, 2008) and several fungal pathogens of plants for evading their hosts' defense systems (Tanaka et al, 2007). In most cases, genetic interactions with SSD1-V are positive, whereas the premature termination alleles (ssd1-d) cause lethality or a more extreme phenotype.…”

“…Despite, or perhaps because of, its impact on so many different cellular functions, SSD1 is a common site of variation in both laboratory strains and natural populations of budding yeast (Wheeler et al, 2003). One possible explanation is that SSD1-V, which encodes the full-length "wildtype" protein, reduces the virulence of Saccharomyces cerevisiae in one mouse model (Wheeler et al, 2003), but SSD1-V is critical in Candida (Gank et al, 2008) and several fungal pathogens of plants for evading their hosts' defense systems (Tanaka et al, 2007).…”

“…These genes show a striking enrichment (Hong et al, 2008) for posttranslational modifiers (p ϭ 10 Ϫ14 ), including 19 kinases and nine histone deacetylases, and genes involved in the cell cycle and cell morphogenesis (p ϭ 10 Ϫ8 ). ssd1 mutants display sensitivity to high osmolarity, caffeine, fungicides¤ and numerous other compounds, which suggests a role for this protein in the maintenance of cell wall integrity (Ibeas et al, 2001;Parsons et al, 2004), but its mechanism of action remains obscure.Despite, or perhaps because of, its impact on so many different cellular functions, SSD1 is a common site of variation in both laboratory strains and natural populations of budding yeast (Wheeler et al, 2003). One possible explanation is that SSD1-V, which encodes the full-length "wildtype" protein, reduces the virulence of Saccharomyces cerevisiae in one mouse model (Wheeler et al, 2003), but SSD1-V is critical in Candida (Gank et al, 2008) and several fungal pathogens of plants for evading their hosts' defense systems (Tanaka et al, 2007).…”

LncRNAs: Bridging environmental sensing and gene expression

“…The natural habitat for S. cerevisiae is fresh and decaying fruit where carbon sources are abundant and diverse. 37,38 These yeast preferentially use glucose as a carbon source, however, they have the ability to switch their metabolic profile to use alternative sugars, a response which is essential to environmental adaptation. 39 This switch involves reprogramming of upwards of 40% of the yeast transcriptome as a result of derepression and transcriptional activation of genes necessary for metabolism of sugars other than glucose.…”