No abstract

epithelial cells (71). Ligand-host cell receptor interactions appear to be complex and strain specific (33). Finally, the host response to C. albicans is also complex. The type of Candida infection depends on the specific host defect. In general, systemic disease is usually an outcome of neutrophil depletion or defects, while cutaneous or mucocutaneous disease is a result of defects in cell-mediated immunity (104). These few examples illustrate the diversity of the organism and point to the highly adapted nature of this pathogen. The purpose of this review is to focus on the cell wall of C. albicans, especially the outer cell surface macromolecules that confer ligandand receptorlike activity on the organism. THE CELL WALL: AN OVERVIEW The cell wall of C. albicans is composed primarily of the polysaccharides mannan, glucan, and chitin. Although the synthesis of the cell wall components is dynamically influenced by growth conditions and metabolic states, the literature contains fairly consistent data regarding the chemical composition of the cell wall. Mannans represent about 15.2 to 22.9% of the yeast cell wall (dry weight), or about 40% of the total cell wall polysaccharide (120). P-1,3-D-Glucans and ,B-1,6-D-glucans account for 47 to 60% by weight of the cell wall (30, 149). Proteins (3, 4, 6, 100, 115, 117, 129) have been reported to account for 6 to 25%, lipids for 1 to 7%, and chitin for 0.6 to 9% by weight of the cell wall (46, 72, 124, 168). The percent compositions of cell walls from yeast cells and germ tubes are relatively similar, although the amounts of alkali-soluble and-insoluble glucans and chitin from C. albicans vary according to the growth form (30). The spatial relationships of these polymers to each other are presented in Fig. 1. Ultrastructural studies of the C. albicans cell wall have indicated a complex microarchitecture. The wall is of variable thickness and is composed of several layers that are revealed by differences in electron density. The number of layers and their morphology are variable and may be related 1

In a previous study, we reported the isolation and characterization of the two-component response regulator SSK1 gene of Candida albicans. This gene is a structural but not a functional homolog of the SSK1 and mcs4 ؉ genes of Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively. In the present study, we have constructed and phenotypically characterized ⌬ssk1 mutants of C. albicans. The results confirmed our previous observation that CaSSK1, unlike SSK1 or mcs4 ؉ , does not regulate cellular responses to either osmotic or oxidative stress. Instead, ⌬ssk1 null strains showed severely reduced hyphal formation on serum agar and were totally defective in hyphal development on other solid media, such as medium 199 (pH 7.5) and Spider medium. In contrast, under conditions of low nitrogen availability on solid media, ⌬ssk1 null strains dramatically hyperinvaded the agar. However, while forming germ tubes and hyphae in liquid media similar to those of the wild type, ⌬ssk1 null strains flocculated in a manner similar to that of ⌬chk1 two-component histidine kinase mutants, which we have previously described. Finally, virulence studies indicated that SSK1 is essential for the pathogenesis of C. albicans, suggesting that the Ssk1p response regulator could be a good target for antifungal therapy.

Candida species and Aspergillus fumigatus were once thought to be relatively benign organisms. However, it is now known that this is not the case - Candida species rank among the top four causes of nosocomial infectious diseases in humans and A. fumigatus is the most deadly mould, often having a 90% mortality rate in immunocompromised transplant recipients. Adaptation to stress, including oxidative stress, is a necessary requisite for survival of these organisms during infection. Here, we describe the latest information on the signalling pathways and target proteins that contribute to oxidant adaptation in C. albicans and A. fumigatus, which has been obtained primarily through the analysis of mutants or inference from genome annotation.

Using a Tn7 transposon library of Candida albicans, we have identified a mutant that exhibited sensitivity in drop plate assays to oxidants such as menadione and hydrogen peroxide. To verify the role of the mutated gene in stress adaptation, null mutants were constructed and phenotypically characterized. Because of its apparent functions in growth and oxidant adaptation, we have named the gene GOA1. Goa1p appears to be unique to the CTG subclade of the Saccharomycotina, including C. albicans. Mutants of C. albicans lacking goa1 (strain GOA31) were more sensitive to 6 mM H 2 O 2 and 0.125 mM menadione than the wild type (wt) or a genereconstituted (GOA32) strain. The sensitivity to oxidants correlated with reduced survival of the GOA31 mutant in human neutrophils and avirulence compared to control strains. Other phenotypes of GOA31 include reduced growth and filamentation in 10% serum, Spider, and SLAD agar media and an inability to form chlamydospores. Since Goa1p has an N-terminal mitochondrion localization site, we also show that green fluorescent protein-tagged Goa1p shows a mitochondrionlike distribution during oxidant or osmotic stress. Further, the inability of GOA31 to grow in medium containing lactate, ethanol, or glycerol as the sole carbon source indicates that the mitochondria are defective in the mutant. To determine how Goa1p contributes to mitochondrial function, we compared the wt, GOA32, and GOA31 strains for mitochondrial electrical membrane potential, respiration, and oxidative phosphorylation. We now show that GOA31, but not the wt or GOA32, had decreased respiration and mitochondrial membrane potential such that mutant cells are unable to drive oxidative phosphorylation. This is the first report in C. albicans of a respiratory defect caused by a loss of mitochondrial membrane potential.