Exophiala dermatitidis is a fungus, often called “black yeast,” found widely in various environments. While usually harmless, it can cause various infections in humans. These range from minor skin issues to severe, life-threatening conditions, especially in individuals with underlying health challenges.
Characteristics and Natural Habitats
Exophiala dermatitidis is a black yeast that can grow as yeast-like cells or filamentous hyphae. Its colonies are slow-growing, brown or black, starting moist and becoming velvety with age. This fungus tolerates extreme conditions, thriving in temperatures from 4 °C to 47 °C and across a broad pH range (2.0–12.0). It can also withstand high salt concentrations (up to 20% NaCl), UV, and radioactive radiation.
It is widely distributed in diverse environments, favoring warm, humid, and hydrocarbon-rich habitats like decaying wood, organic debris, and aquatic biofilms. The fungus is also often found in human-made indoor environments, especially those with water sources and heat. Common indoor locations include dishwashers, washing machine seals, bathrooms, and saunas, where heat and moisture support its growth.
Health Conditions It Causes
Exophiala dermatitidis causes infections ranging from superficial skin conditions to deep-seated and disseminated diseases. Superficial infections often appear as skin lesions, such as mycetoma, characterized by tumor-like growths with black granules, or chromoblastomycosis, a skin infection with pigmented sclerotic bodies. Onychomycosis, a fungal infection of the nails, also represents a superficial manifestation.
Moving deeper, subcutaneous infections like phaeohyphomycosis can develop, presenting as localized lesions under the skin. This invasive disease can extend from cutaneous and subcutaneous involvement to systemic spread within internal organs. Pulmonary infections are common deep infections, often from inhaling fungal spores. These respiratory issues range from non-invasive colonization, especially in those with chronic lung conditions, to severe, invasive pneumonia.
The fungus can also cause serious systemic or disseminated infections affecting various internal organs. Brain abscesses are a severe, often fatal, form of phaeohyphomycosis, as Exophiala dermatitidis tends to infect nervous tissue. Fungemia, fungi in the bloodstream, is another severe manifestation leading to widespread infection and organ dysfunction. These systemic infections have significant morbidity and a high fatality rate, sometimes reaching 40%.
Who is Most Susceptible
Individuals with weakened immune systems are at a higher risk for Exophiala dermatitidis infections. This includes patients undergoing organ transplantation or chemotherapy, those with hematological malignancies, and individuals living with HIV/AIDS. Their compromised immune defenses make them less able to fight off opportunistic pathogens like this fungus.
People with chronic lung conditions are also susceptible to respiratory colonization and infection. Patients with cystic fibrosis (CF) often have Exophiala dermatitidis frequently isolated from their bronchial secretions, which can accelerate lung function decline. The impaired mucociliary clearance and mucus retention in CF lungs create an environment conducive to fungal growth.
A genetic predisposition, specifically a CARD9 protein deficiency, links to increased susceptibility to severe Exophiala dermatitidis infections, including central nervous system involvement. This deficiency impairs the body’s immune response to fungi. Other risk factors include occupational exposure to contaminated environments, such as in silviculture, or pre-existing skin trauma allowing fungal entry.
Diagnosis and Treatment Approaches
Diagnosing Exophiala dermatitidis infections typically involves a combination of laboratory methods. Fungal cultures from affected tissues or fluids, like sputum or biopsy samples, are a primary diagnostic approach, with growth observed on Sabouraud dextrose agar at 35 °C. Microscopic examination of stained samples reveals brown-pigmented hyphae and yeast-like cells, characteristic of dematiaceous fungi.
For definitive identification, molecular techniques are used. Amplification and sequencing of the internal transcribed spacer (ITS) region of ribosomal DNA is a reliable method for species-level identification. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) is also a rapid diagnostic tool, though confirmatory ITS sequencing is often recommended.
Treatment primarily uses antifungal medications, though no standardized guidelines exist due to case rarity. Azole antifungals, such as voriconazole and itraconazole, are frequently used and show good activity. Amphotericin B is another antifungal agent for severe or disseminated cases. Susceptibility testing is important to determine the most effective drug, as the fungus can exhibit varied resistance patterns. For localized lesions or abscesses, surgical intervention may be necessary to remove infected tissue and improve outcomes.
Prevention Strategies
Minimizing exposure to Exophiala dermatitidis helps prevent infections, especially for susceptible individuals. Good personal hygiene, including regular handwashing, is a general preventive measure. Proper wound care is also important, as skin trauma can serve as an entry point for the fungus.
Individuals with compromised immune systems should be careful to avoid prolonged contact with potentially contaminated water or soil. This includes limiting activities in damp environments where the fungus thrives. Regular cleaning and maintenance of household appliances that harbor moisture are also recommended. Dishwashers, humidifiers, and saunas can be sources of Exophiala dermatitidis, so keeping these environments clean reduces fungal growth and exposure.