New insights into immunity to skin fungi shape our understanding of health and disease

Conversation between skin microbiota and the host: from early life to adulthood

Host life is inextricably linked to commensal microbiota, which play a crucial role in maintaining homeostasis and immune activation. A diverse array of commensal microbiota on the skin interacts with the host, influencing the skin physiology in various ways. Early-life exposure to commensal microbiota has long-lasting effects, and disruption of the epidermal barrier or transient exposure to these microorganisms can lead to skin dysbiosis and inflammation. Several commensal skin microbiota have the potential to function as either commensals or pathogens, both influencing and being influenced by the pathogenesis of skin inflammatory diseases. Here we explore the impact of various commensal skin microbiota on the host and elucidate the interactions between skin microbiota and host systems. A deeper understanding of these interactions may open new avenues for developing effective strategies to address skin diseases.

A greener side of health care: Revisiting phytomedicine against the human fungal pathogen Malassezia

Malassezia species are commensal fungi residing on the skin and in the gut of humans and animals. Yet, under certain conditions, they become opportunistic pathogens leading to various clinical conditions including dermatological disorders. The emergence of drug resistance and adverse effects associated with conventional antifungal agents has propelled the search for alternative treatments, among which phytomedicine stands out prominently. Phytochemicals, including phenolic acids, flavonoids, and terpenoids, demonstrate potential antifungal activity against Malassezia by inhibiting its growth, adhesion, and biofilm formation. Furthermore, the multifaceted therapeutic properties of phytomedicine (including anti-fungal and, antioxidant properties) contribute to its efficacy in alleviating symptoms associated with Malassezia infections. Despite these promising prospects, several challenges hinder the widespread adoption of phytomedicine in clinical practice mostly since the mechanistic studies and controlled experiments to prove efficacy have not been done. Issues include standardization of herbal extracts, variable bioavailability, and limited clinical evidence. Hence, proper regulatory constraints necessitate comprehensive research endeavors and regulatory frameworks to harness the full therapeutic potential of phytomedicine. In conclusion, while phytomedicine holds immense promise as an alternative or adjunctive therapy against Malassezia, addressing these challenges is imperative to optimize its efficacy and ensure its integration into mainstream medical care. In this review we provide an update on the potential phytomedicines in combating Malassezia-related ailments, emphasizing its diverse chemical constituents and mechanisms of action.

Atopic dermatitis: new insights into a common condition

Atopic dermatitis is one of the most common conditions managed in the community world-wide. Atopic dermatitis is under intense scrutiny by academic researchers, not to mention drug companies, enabling regular research in the area. Dermatologists have a growing appreciation of the complex causes of this common condition.

Commensal microbe regulation of skin cells in disease

Human skin is the host to various commensal microbes that constitute a substantial microbial community. The reciprocal communication between these microbial inhabitants and host cells upholds both the morphological and functional attributes of the skin layers, contributing indispensably to microenvironmental and tissue homeostasis. Thus, disruption of the skin barrier or imbalances in the microbial communities can exert profound effects on the behavior of host cells. This influence, mediated by the microbes themselves or their metabolites, manifests in diverse outcomes. In this review, we examine existing knowledge to provide insight into the nuanced behavior exhibited by the microbiota on skin cells in health and disease states. These interactions provide insight into potential cellular targets for future microbiota-based therapies to prevent and treat skin disease.

Epidermal barrier impairment predisposes for excessive growth of the allergy-associated yeast Malassezia on murine skin

BACKGROUND

The skin barrier is vital for protection against environmental threats including insults caused by skin-resident microbes. Dysregulation of this barrier is a hallmark of atopic dermatitis (AD) and ichthyosis, with variable consequences for host immune control of colonizing commensals and opportunistic pathogens. While Malassezia is the most abundant commensal fungus of the skin, little is known about the host control of this fungus in inflammatory skin diseases.

METHODS

In this experimental study, MC903-treated mice were colonized with Malassezia spp. to assess the host-fungal interactions in atopic dermatitis. Additional murine models of AD and ichthyosis, including tape stripping, K5-Nrf2 overexpression and flaky tail mice, were employed to confirm and expand the findings. Skin fungal counts were enumerated. High parameter flow cytometry was used to characterize the antifungal response in the AD-like skin. Structural and functional alterations in the skin barrier were determined by histology and transcriptomics of bulk skin. Finally, differential expression of metabolic genes in Malassezia in atopic and control skin was quantified.

RESULTS

Malassezia grows excessively in AD-like skin. Fungal overgrowth could, however, not be explained by the altered immune status of the atopic skin. Instead, we found that by upregulating key metabolic genes in the altered cutaneous niche, Malassezia acquired enhanced fitness to efficiently colonise the impaired skin barrier.

CONCLUSIONS

This study provides evidence that structural and metabolic changes in the dysfunctional epidermal barrier environment provide increased accessibility and an altered lipid profile, to which the lipid-dependent yeast adapts for enhanced nutrient assimilation. Our findings reveal fundamental insights into the implication of the mycobiota in the pathogenesis of common skin barrier disorders.

Water-filtered infrared A irradiation exerts antifungal effects on the skin fungus Malassezia

Many common skin diseases are associated with changes in the microbiota. This applies for the commensal yeast Malassezia, which is linked to a wide range of skin disorders ranging from mild dandruff to severe seborrheic and atopic dermatitis, all of which have a detrimental impact on the individuals' quality of life. While antifungal medications offer relief in many cases, the challenges of disease recurrence and the emergence of resistance to the limited range of available antifungal drugs poses a pressing need for innovative therapeutic options. Here we examined the activity of water-filtered infrared A (wIRA) irradiation against Malassezia. wIRA's antimicrobial and wound healing properties make it an attractive option for localized, non-invasive, and contact-free treatment of superficial skin infections. Irradiation of Malassezia furfur with wIRA (570-1400 nm) resulted in a reduction of the yeast's metabolic activity. When put in contact with immune cells, wIRA-irradiated M. furfur was recovered at lower counts than non-irradiated M. furfur. Likewise, wIRA irradiation of M. furfur put in contact with keratinocytes, the primary host interface of the fungus in the skin, reduced the fungal counts, while the keratinocytes were not affected by the irradiation. The combination of wIRA with the photosensitizer methyl aminolevulinate exerted an additional antifungal effect on M. furfur, irrespective of the presence or absence of keratinocytes, suggesting an enhancement of the treatment effect when used in combination. These findings suggest that wIRA holds promise as a potential therapy for skin disorders associated with Malassezia.

Mechanisms of pathogenicity for the emerging fungus Candida auris

Candida auris recently emerged as an urgent public health threat, causing outbreaks of invasive infections in healthcare settings throughout the world. This fungal pathogen persists on the skin of patients and on abiotic surfaces despite antiseptic and decolonization attempts. The heightened capacity for skin colonization and environmental persistence promotes rapid nosocomial spread. Following skin colonization, C. auris can gain entrance to the bloodstream and deeper tissues, often through a wound or an inserted medical device, such as a catheter. C. auris possesses a variety of virulence traits, including the capacity for biofilm formation, production of adhesins and proteases, and evasion of innate immune responses. In this review, we highlight the interactions of C. auris with the host, emphasizing the intersection of laboratory studies and clinical observations.