Hortaea Werneckii

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Introduction

Hortaea werneckii (H. werneckii) is an ascomycetous yeast causing a rare superficial and non-invasive skin infection tinea nigra. It is also known as Phaeoannellomyces werneckii and Exophiala werneckii previously. It is a dematiaceous and polymorphic fungus that can withstand and adapt to life in an environment with up to almost saturated NaCl solution, while also thriving in environments without NaCl. H. werneckii belongs to genus Hortaea, family Teratosphaeriaceae, order Capnodiales, class Dothideomycetes, and division Ascomycota.

Distribution

The fungus belongs to the black yeast group and can be isolated from different marine habitats and soil samples because of its remarkable halotolerance. It mainly inhabits the soil, particularly being much more prevalent in warm, humid climates of the world such as the Central and South Americas, Africa, and Asia.

Mycology of H. werneckii

H. werneckii, identified as an extreme halotolerant fungus, can switch between two main life phases: yeast phase and hyphal phase.

• Macroscopically, the colonies of H. werneckii grow slowly with initially pale in color, moist, shiny, and yeast-like, while becoming velvety, olive black, and are covered with a thin layer of mycelium over time. The color is black from the opposite of the medium.
• Microscopically, H. werneckii colonies are composed of brown to dark olivaceous, septate hyphal, yeast-like conidia, and chlamydospores. The yeast-like conidia are two-celled, cylindrical to spindle-shaped that only observed in the early phase of the colony development. They are hyaline initially and become pale brown in time with a round end and a tapered and elongated annellidic neck part. The conidia taper towards the ends to form an annellide and produce new annelloconidia. And the annelloconidia are formed at intercalary and lateral annellidic points along the hyphae. Septate, thick-walled, and brown hyphae are formed with the colony development.

Fig.1 Macroscopic and microscopic phenotypes of H. werneckii. (Chen, 2012)

Salt-tolerance Mechanisms of H. werneckii

It has been previously indicated that glycerol is the major compatible solute of H. werneckii. Others, including erythritol, arabitol, and mannitol, are also present in the exponentially growing cells at optimal growth salinities. When exposed to maximal salinities, only glycerol and erythritol are present in H. werneckii, and their decrease is correlated with the severity of hypoosmotic shock. Besides erythritol and glycerol, mycosporine-glutaminol-glucoside also might act as a complementary compatible solute of H. werneckii. Localization and content of melanin in H. werneckii cell-wall vary with the change of salt concentration. Melanized cell wall plays an important role in the effective retention of glycerol and typical symptom brownish black lesions of tinea nigra.

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Reference

1. Chen, J.; et al. Identification of Hortaea werneckii Isolated from Mangrove Plant Aegiceras comiculatum Based on Morphology and rDNA Sequences. Mycopathologia. 2012, 174(5-6):457-66.

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