Antigenicity of Cell Wall Mannans of Candida albicans and Candida stellatoidea Cultured at High Temperatures in BACTEC Medium

Fungal Cell Wall Proteins and Signaling Pathways Form a Cytoprotective Network to Combat Stresses

Candida species are part of the normal flora of humans, but once the immune system of the host is impaired and they escape from commensal niches, they shift from commensal to pathogen causing candidiasis. Candida albicans remains the primary cause of candidiasis, accounting for about 60% of the global candidiasis burden. The cell wall of C. albicans and related fungal pathogens forms the interface with the host, gives fungal cells their shape, and also provides protection against stresses. The cell wall is a dynamic organelle with great adaptive flexibility that allows remodeling, morphogenesis, and changes in its components in response to the environment. It is mainly composed of the inner polysaccharide rich layer (chitin, and β-glucan) and the outer protein coat (mannoproteins). The highly glycosylated protein coat mediates interactions between C. albicans cells and their environment, including reprograming of wall architecture in response to several conditions, such as carbon source, pH, high temperature, and morphogenesis. The mannoproteins are also associated with C. albicans adherence, drug resistance, and virulence. Vitally, the mannoproteins contribute to cell wall construction and especially cell wall remodeling when cells encounter physical and chemical stresses. This review describes the interconnected cell wall integrity (CWI) and stress-activated pathways (e.g., Hog1, Cek1, and Mkc1 mediated pathways) that regulates cell wall remodeling and the expression of some of the mannoproteins in C. albicans and other species. The mannoproteins of the surface coat is of great importance to pathogen survival, growth, and virulence, thus understanding their structure and function as well as regulatory mechanisms can pave the way for better management of candidiasis.

Physiologically Relevant Alternative Carbon Sources Modulate Biofilm Formation, Cell Wall Architecture, and the Stress and Antifungal Resistance of Candida glabrata

Flexibility in carbon metabolism is pivotal for the survival and propagation of many human fungal pathogens within host niches. Indeed, flexible carbon assimilation enhances pathogenicity and affects the immunogenicity of Candida albicans. Over the last decade, Candida glabrata has emerged as one of the most common and problematic causes of invasive candidiasis. Despite this, the links between carbon metabolism, fitness, and pathogenicity in C. glabrata are largely unexplored. Therefore, this study has investigated the impact of alternative carbon metabolism on the fitness and pathogenic attributes of C. glabrata. We confirm our previous observation that growth on carbon sources other than glucose, namely acetate, lactate, ethanol, or oleate, attenuates both the planktonic and biofilm growth of C. glabrata, but that biofilms are not significantly affected by growth on glycerol. We extend this by showing that C. glabrata cells grown on these alternative carbon sources undergo cell wall remodeling, which reduces the thickness of their β-glucan and chitin inner layer while increasing their outer mannan layer. Furthermore, alternative carbon sources modulated the oxidative stress resistance of C. glabrata as well as the resistance of C. glabrata to an antifungal drug. In short, key fitness and pathogenic attributes of C. glabrata are shown to be dependent on carbon source. This reaffirms the perspective that the nature of the carbon sources available within specific host niches is crucial for C. glabrata pathogenicity during infection.

Impact of the Environment upon the Candida albicans Cell Wall and Resultant Effects upon Immune Surveillance

The fungal cell wall is an essential organelle that maintains cellular morphology and protects the fungus from environmental insults. For fungal pathogens such as Candida albicans, it provides a degree of protection against attack by host immune defences. However, the cell wall also presents key epitopes that trigger host immunity and attractive targets for antifungal drugs. Rather than being a rigid shield, it has become clear that the fungal cell wall is an elastic organelle that permits rapid changes in cell volume and the transit of large liposomal particles such as extracellular vesicles. The fungal cell wall is also flexible in that it adapts to local environmental inputs, thereby enhancing the fitness of the fungus in these microenvironments. Recent evidence indicates that this cell wall adaptation affects host-fungus interactions by altering the exposure of major cell wall epitopes that are recognised by innate immune cells. Therefore, we discuss the impact of environmental adaptation upon fungal cell wall structure, and how this affects immune recognition, focussing on C. albicans and drawing parallels with other fungal pathogens.

Dressed to impress: impact of environmental adaptation on the Candida albicans cell wall

Candida albicans is an opportunistic fungal pathogen of humans causing superficial mucosal infections and life‐threatening systemic disease. The fungal cell wall is the first point of contact between the invading pathogen and the host innate immune system. As a result, the polysaccharides that comprise the cell wall act as pathogen associated molecular patterns, which govern the host–pathogen interaction. The cell wall is dynamic and responsive to changes in the external environment. Therefore, the host environment plays a critical role in regulating the host–pathogen interaction through modulation of the fungal cell wall. This review focuses on how environmental adaptation modulates the cell wall structure and composition, and the subsequent impact this has on the innate immune recognition of C. albicans.

Chemical structure of the cell-wall mannan of Candida albicans serotype A and its difference in yeast and hyphal forms

The structure of the cell-wall mannan from the J-1012 (serotype A) strain of the polymorphic yeast Candida albicans was determined by acetolysis under mild conditions followed by HPLC and sequential NMR experiments. The serotype A mannan contained beta-1,2-linked mannose residues attached to alpha-1,3-linked mannose residues and alpha-1,6-linked branching mannose residues. Using a beta-1,2-mannosyltransferase, we synthesized a three-beta-1,2-linkage-containing mannoheptaose and used it as a reference oligosaccharide for 1H-NMR assignment. On the basis of the results obtained, we derived an additivity rule for the 1H-NMR chemical shifts of the beta-1,2-linked mannose residues. The morphological transformation of Candida cells from the yeast form to the hyphal form induced a significant decrease in the phosphodiesterified acid-labile beta-1,2-linked manno-oligosaccharides, whereas the amount of acid-stable beta-1,2 linkage-containing side chains did not change. These results suggest that the Candida mannan in candidiasis patients contains beta-1,2-linked mannose residues and that they behave as a target of the immune system.