Candida albicans commensalism in the oral mucosa is favoured by limited virulence and metabolic adaptation

As part of the human microbiota, the fungus Candida albicans colonizes the oral cavity and other mucosal surfaces of the human body. Commensalism is tightly controlled by complex interactions of the fungus and the host to preclude fungal elimination but also fungal overgrowth and invasion, which can result in disease. As such, defects in antifungal T cell immunity render individuals susceptible to oral thrush due to interrupted immunosurveillance of the oral mucosa. The factors that promote commensalism and ensure persistence of C. albicans in a fully immunocompetent host remain less clear. Using an experimental model of C. albicans oral colonization in mice we explored fungal determinants of commensalism in the oral cavity. Transcript profiling of the oral isolate 101 in the murine tongue tissue revealed a characteristic metabolic profile tailored to the nutrient poor conditions in the stratum corneum of the epithelium where the fungus resides. Metabolic adaptation of isolate 101 was also reflected in enhanced nutrient acquisition when grown on oral mucosa substrates. Persistent colonization of the oral mucosa by C. albicans also correlated inversely with the capacity of the fungus to induce epithelial cell damage and to elicit an inflammatory response. Here we show that these immune evasive properties of isolate 101 are explained by a strong attenuation of a number of virulence genes, including those linked to filamentation. De-repression of the hyphal program by deletion or conditional repression of NRG1 abolished the commensal behaviour of isolate 101, thereby establishing a central role of this factor in the commensal lifestyle of C. albicans in the oral niche of the host.

The oral microbiota represents an important part of the human microbiota and includes several hundreds to several thousands of bacterial and fungal species. One of the most prominent fungus colonizing the oral cavity is the yeast Candida albicans. While the presence of C. albicans usually remains unnoticed, the fungus can under certain circumstances cause lesions on the lining of the mouth referred to as oral thrush or contribute to other common oral diseases such as caries. Maintaining C. albicans commensalism in the oral mucosa is therefore of utmost importance for oral health and overall wellbeing. While overt fungal growth and disease is limited by immunosurveillance mechanisms during homeostasis, C. albicans strives to survive and evades elimination from the host. Here, we show that while commensalism in the oral cavity is characterized by a restricted fungal virulence and hyphal program, enforcing filamentation in a commensal isolate is sufficient for driving pathogenicity and fungus-induced inflammation in the oral mucosa thwarting persistent colonization. Our results further support a critical role for specialized nutrient acquisition allowing the fungus to thrive in the nutrient poor environment of the squamous epithelium. Together, this work revealed key determinants of C. albicans commensalism in the oral niche.

The histone chaperone HIR maintains chromatin states to control nitrogen assimilation and fungal virulence

Adaptation to changing environments and immune evasion is pivotal for fitness of pathogens. Yet, the underlying mechanisms remain largely unknown. Adaptation is governed by dynamic transcriptional re-programming, which is tightly connected to chromatin architecture. Here, we report a pivotal role for the HIR histone chaperone complex in modulating virulence of the human fungal pathogen Candida albicans. Genetic ablation of HIR function alters chromatin accessibility linked to aberrant transcriptional responses to protein as nitrogen source. This accelerates metabolic adaptation and increases the release of extracellular proteases, which enables scavenging of alternative nitrogen sources. Furthermore, HIR controls fungal virulence, as HIR1 deletion leads to differential recognition by immune cells and hypervirulence in a mouse model of systemic infection. This work provides mechanistic insights into chromatin-coupled regulatory mechanisms that fine-tune pathogen gene expression and virulence. Furthermore, the data point toward the requirement of refined screening approaches to exploit chromatin modifications as antifungal strategies.