Susceptibility of gnotobiotic transgenic mice (Tgepsilon26) with combined deficiencies in natural killer cells and T cells to wild-type and hyphal signalling-defective mutants of Candida albicans

Contrasting Strategies: Human Eukaryotic Versus Bacterial Microbiome Research

Most discussions of human microbiome research have focused on bacterial investigations and findings. Our target is to understand how human eukaryotic microbiome research is developing, its potential distinctiveness, and how problems can be addressed. We start with an overview of the entire eukaryotic microbiome literature (578 papers), show tendencies in the human‐based microbiome literature, and then compare the eukaryotic field to more developed human bacterial microbiome research. We are particularly concerned with problems of interpretation that are already apparent in human bacterial microbiome research (e.g. disease causality, probiotic interventions, evolutionary claims). We show where each field converges and diverges, and what this might mean for progress in human eukaryotic microbiome research. Our analysis then makes constructive suggestions for the future of the field.

Effect of Antifungal Treatment in a Diet-Based Murine Model of Disseminated Candidiasis Acquired via the Gastrointestinal Tract

Candida albicans, normally found as a commensal in the gut, is a major human fungal pathogen responsible for both mucosal and systemic infections in a wide variety of immunocompromised individuals, including cancer patients and organ transplant recipients. The gastrointestinal tract represents a major portal of entry for the establishment of disseminated candidiasis in many of these individuals. Here we report the development of a diet-based mouse model for disseminated candidiasis acquired via the gastrointestinal tract. Using this model, as well as an appropriate immunosuppression regimen, we demonstrate that dissemination of C. albicans from the gastrointestinal tract can result in mortality within 30 days postinfection. We also show a significant increase in fungal burden in systemic organs, but not gastrointestinal tract organs, upon immunosuppression. Importantly, we demonstrate that the administration of two widely used antifungals, fluconazole and caspofungin, either pre- or postimmunosuppression, significantly reduces fungal burdens. This model should prove to be of significant value for testing the ability of both established and experimental therapeutics to inhibit C. albicans dissemination from the gastrointestinal tract in an immunocompromised host as well as the subsequent mortality that can result from disseminated candidiasis.

The game theory of Candida albicans colonization dynamics reveals host status-responsive gene expression

Background

The fungal pathogen Candida albicans colonizes the gastrointestinal (GI) tract of mammalian hosts as a benign commensal. However, in an immunocompromised host, the fungus is capable of causing life-threatening infection. We previously showed that the major transcription factor Efg1p is differentially expressed in GI-colonizing C. albicans cells dependent on the host immune status. To understand the mechanisms that underlie this host-dependent differential gene expression, we utilized mathematical modeling to dissect host-pathogen interactions. Specifically, we used principles of evolutionary game theory to study the mechanism that governs dynamics of EFG1 expression during C. albicans colonization.

Results

Mathematical modeling predicted that down-regulation of EFG1 expression within individual fungal cells occurred at different average rates in different hosts. Rather than using relatively transient signaling pathways to adapt to a new environment, we demonstrate that C. albicans overcomes the host defense strategy by modulating the activity of diverse fungal histone modifying enzymes that control EFG1 expression.

Conclusion

Based on our modeling and experimental results we conclude that C. albicans cells sense the local environment of the GI tract and respond to differences by altering EFG1 expression to establish optimal survival strategies. We show that the overall process is governed via modulation of epigenetic regulators of chromatin structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0268-1) contains supplementary material, which is available to authorized users.

The APSES transcription factor Efg1 is a global regulator that controls morphogenesis and biofilm formation in Candida parapsilosis

Efg1 (a member of the APSES family) is an important regulator of hyphal growth and of the white-to-opaque transition in Candida albicans and very closely related species. We show that in Candida parapsilosis Efg1 is a major regulator of a different morphological switch at the colony level, from a concentric to smooth morphology. The rate of switching is at least 20-fold increased in an efg1 knockout relative to wild type. Efg1 deletion strains also have reduced biofilm formation, attenuated virulence in an insect model, and increased sensitivity to SDS and caspofungin. Biofilm reduction is more dramatic in in vitro than in in vivo models. An Efg1 paralogue (Efh1) is restricted to Candida species, and does not regulate concentric-smooth phenotype switching, biofilm formation or stress response. We used ChIP-seq to identify the Efg1 regulon. A total of 931 promoter regions bound by Efg1 are highly enriched for transcription factors and regulatory proteins. Efg1 also binds to its own promoter, and negatively regulates its expression. Efg1 targets are enriched in binding sites for 93 additional transcription factors, including Ndt80. Our analysis suggests that Efg1 has an ancient role as regulator of development in fungi, and is central to several regulatory networks.

Murine models of Candida gastrointestinal colonization and dissemination

Ninety-five percent of infectious agents enter through exposed mucosal surfaces, such as the respiratory and gastrointestinal (GI) tracts. The human GI tract is colonized with trillions of commensal microbes, including numerous Candida spp. Some commensal microbes in the GI tract can cause serious human infections under specific circumstances, typically involving changes in the gut environment and/or host immune conditions. Therefore, utilizing animal models of fungal GI colonization and dissemination can lead to significant insights into the complex pathophysiology of transformation from a commensal organism to a pathogen and host-pathogen interactions. This paper will review the methodologic approaches used for modeling GI colonization versus dissemination, the insights learned from these models, and finally, possible future directions using these animal modeling systems.

Inflammation and gastrointestinal Candida colonization

Candida organisms commonly colonize the human gastrointestinal tract as a component of the resident microbiota. Their presence is generally benign. Recent studies, however, show that high level Candida colonization is associated with several diseases of the gastrointestinal tract. Further, results from animal models argue that Candida colonization delays healing of inflammatory lesions and that inflammation promotes colonization. These effects may create a vicious cycle in which low-level inflammation promotes fungal colonization and fungal colonization promotes further inflammation. Both inflammatory bowel disease and gastrointestinal Candida colonization are associated with elevated levels of the pro-inflammatory cytokine IL-17. Therefore, effects on IL-17 levels may underlie the ability of Candida colonization to enhance inflammation. Because Candida is a frequent colonizer, these effects have the potential to impact many people.

The Ras/cAMP/PKA signaling pathway and virulence in Candida albicans

Candidiasis is the most common cause of fungal infections, and the majority of these are caused by Candida albicans. The protean pathogenic potential of C. albicans includes the capacity to infect diverse mucosal and epidermal surfaces as well as to disseminate via the bloodstream to internal organs, potentially causing system failure in cases of severe immunosuppression. Many environmental niches in the host may be invaded by C. albicans through modulation of gene expression patterns while changing morphology between yeast and hyphal growth forms. The Ras/cAMP/PKA signaling pathway has attracted particular attention for its role in promoting hyphal growth and because of its importance in virulence. Here, we present an overview of the components of the pathway and their functions, how the pathway may be activated in human hosts and recent updates regarding the role of Ras/cAMP/PKA signaling in virulence.

A URA3 null mutant of Candida albicans (CAI-4) causes oro-oesophageal and gastric candidiasis and is lethal for gnotobiotic, transgenic mice (Tgepsilon26) that are deficient in both natural killer and T cells

Current data suggest that functional URA3 genes are necessary for the full pathogenesis of Candida albicans. Herein it is shown that a putatively avirulent URA3/URA3 null mutant of C. albicans (CAI-4) can colonize the murine alimentary tract, invade oro-oesophageal and gastric tissues with yeasts and hyphae, evoke a granulocyte-dominated inflammatory response, and kill transgenic mice that are deficient for both natural killer cells and T cells. Because C. albicans-colonized (gnotobiotic) mice lack a viable prokaryotic microbiota, this study also demonstrates that the gut microbiome is not required to supply the mutant's nutritional needs. The gnotobiotic murine model described herein can be used to assess the capacity of C. albicans mutants to colonize and infect cutaneous, mucosal and systemic tissues and kill the susceptible host via a clinically common, natural route of infection; namely the alimentary tract.

Animal models of mucosal Candida infection

Rodent models of oral, vaginal and gastrointestinal Candida infection are described and discussed in terms of their scientific merits. The common feature of all experimental mucosal Candida infections is the need for some level of host immunocompromise or exogenous treatment to ensure quantitatively reproducible disease. A growing literature describes the contributions of such candidiasis models to our understanding of certain aspects of fungal virulence and host response to mucosal Candida albicans challenge. Evidence to date shows that T-lymphocyte responses dominate host immune defences to oral and gastrointestinal challenge, while other, highly compartmentalized responses defend vaginal surfaces. By contrast the study of C. albicans virulence factors in mucosal infection models has only begun to unravel the complex of attributes required to define the difference between strongly and weakly muco-invasive strains.