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Comparative Ecology of Capsular Exophiala Species Causing Disseminated Infection in Humans

Yinggai Song^1,2,3,4, Wendy W J Laureijssen-van de Sande⁵, Leandro F Moreno⁴

¹Department of Dermatology, Peking University First Hospital, Beijing, China.

Frontiers in Microbiology

|January 10, 2018

Abstract:

Exophiala spinifera and Exophiala dermatitidis (Fungi: Chaetothyriales) are black yeast agents potentially causing disseminated infection in apparently healthy humans. They are the only Exophiala species producing extracellular polysaccharides around yeast cells. In order to gain understanding of eventual differences in intrinsic virulence of the species, their clinical profiles were compared and found to be different, suggesting pathogenic strategies rather than coincidental opportunism. Ecologically relevant factors were compared in a model set of strains of both species, and significant differences were found in clinical and environmental preferences, but virulence, tested in Galleria mellonella larvae, yielded nearly identical results. Virulence factors, i.e., melanin, capsule and muriform cells responded in opposite direction under hydrogen peroxide and temperature stress and thus were inconsistent with their hypothesized role in survival of phagocytosis. On the basis of physiological profiles, possible natural habitats of both species were extrapolated, which proved to be environmental rather than animal-associated. Using comparative genomic analyses we found differences in gene content related to lipid metabolism, cell wall modification and polysaccharide capsule production. Despite the fact that both species cause disseminated infections in apparently healthy humans, it is concluded that they are opportunists rather than pathogens.

Insights

Black yeast fungi, Exophiala spinifera and Exophiala dermatitidis, cause disseminated infections but are considered opportunists, not true pathogens. Despite differing clinical profiles, their virulence and environmental preferences suggest distinct ecological strategies.

Area of Science:

Medical Mycology
Fungal Pathogenesis
Genomics

Background:

Exophiala spinifera and Exophiala dermatitidis are black yeast fungi capable of causing disseminated infections in humans.
These species are unique within the Exophiala genus for producing extracellular polysaccharides, suggesting potential virulence factors.

Purpose of the Study:

To investigate the differences in intrinsic virulence and pathogenic strategies between Exophiala spinifera and Exophiala dermatitidis.
To compare ecological factors and virulence determinants to understand their clinical behavior.

Main Methods:

Comparative analysis of clinical profiles and ecological preferences of Exophiala species.
Virulence testing in Galleria mellonella larvae.
Assessment of virulence factors (melanin, capsule, muriform cells) under stress conditions.
Comparative genomic analysis focusing on lipid metabolism, cell wall, and capsule production.

Main Results:

Clinical profiles differed, suggesting distinct pathogenic strategies, but virulence in Galleria mellonella larvae was similar.
Virulence factors showed inconsistent responses to stress, questioning their role in phagocytosis survival.
Physiological profiles indicated environmental, rather than animal-associated, natural habitats.
Genomic analysis revealed differences in genes related to lipid metabolism, cell wall, and polysaccharide capsule.

Conclusions:

Exophiala spinifera and Exophiala dermatitidis exhibit distinct ecological and clinical behaviors, despite similar virulence in experimental models.
Their stress responses and genetic makeup suggest opportunistic behavior rather than inherent pathogenicity.
Both species are likely environmental fungi that can opportunistically infect humans, particularly those who are immunocompromised.

Read full analysis

Keywords:

Exophiala species Galleria mellonella black yeast capsule opportunism pathogenicity physiology virulence

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