Dematiaceous Molds

These fungi are characterized by their dark pigmentation, which is due to the presence of melanin in their cell walls. We’ll cover the key genera – Alternaria, Cladosporium, and Fonsecaea – along with their disease states, transmission, and lab identification methods

Dematiaceous Molds

Definition: Dematiaceous molds are a diverse group of filamentous fungi characterized by dark-pigmented hyphae and conidia
Melanin: The dark pigmentation is due to the presence of melanin in their cell walls, which provides protection from UV radiation and may contribute to their virulence
Ubiquitous: They are found ubiquitously in the environment, including soil, decaying vegetation, and air
Opportunistic Pathogens: Many dematiaceous molds are opportunistic pathogens, meaning they typically cause infections in individuals with weakened immune systems
Key Genera: The most common genera causing human infections include Alternaria, Cladosporium, Fonsecaea, Exophiala, Curvularia, and Bipolaris
Disease: The specific diseases caused by dematiaceous molds vary depending on the genus and the immune status of the host

Alternaria spp.

Etiology: Alternaria is a genus of dematiaceous molds that is commonly found in the environment and can cause a variety of infections, including skin infections, sinusitis, and asthma
Epidemiology
- Found worldwide in soil, decaying vegetation, and air
- Infections are acquired through inhalation of spores or direct inoculation into the skin
- Risk factors:
  - Exposure to outdoor environments
  - Immunosuppression
  - Trauma
  - Allergies
Transmission
- Inhalation of spores
- Direct inoculation into the skin
Disease States
- Allergic Reactions: Alternaria is a common cause of allergic rhinitis (hay fever) and asthma
- Skin Infections: Can cause cutaneous or subcutaneous infections, particularly in immunocompromised individuals
- Sinusitis: Infection of the sinuses
- Keratitis: Infection of the cornea
- Pneumonia: Rare, but can occur in immunocompromised individuals
Laboratory Diagnosis
- Specimen Collection
  - Skin Scrapings or Biopsy: From skin lesions
  - Sinus Aspirate: For suspected sinusitis
  - Sputum or BAL: For suspected pneumonia
  - Corneal Scrapings: For suspected keratitis
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of skin scrapings or other samples to visualize dark-pigmented, septate hyphae
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Rapid growth at 25-30°C, forming colonies with a dark brown to black color and a woolly or cottony texture
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic conidia (large, multi-celled, with both transverse and longitudinal septa)
  - Conidia often have a “drumstick” or “club-shaped” appearance
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification

Cladosporium spp.

Etiology: Cladosporium is a genus of dematiaceous molds that is commonly found in the environment and can cause a variety of infections, including skin infections, sinusitis, and allergic reactions
Epidemiology
- Found worldwide in soil, decaying vegetation, and air
- Infections are acquired through inhalation of spores or direct inoculation into the skin
- Risk factors:
  - Exposure to outdoor environments
  - Immunosuppression
  - Trauma
  - Allergies
Transmission
- Inhalation of spores
- Direct inoculation into the skin
Disease States
- Allergic Reactions: Cladosporium is a common cause of allergic rhinitis and asthma
- Skin Infections: Can cause cutaneous or subcutaneous infections, particularly in immunocompromised individuals
- Sinusitis: Infection of the sinuses
- Keratitis: Infection of the cornea
- Chromoblastomycosis: A chronic, localized infection of the skin and subcutaneous tissue, characterized by warty nodules
Laboratory Diagnosis
- Specimen Collection
  - Skin Scrapings or Biopsy: From skin lesions
  - Sinus Aspirate: For suspected sinusitis
  - Corneal Scrapings: For suspected keratitis
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of skin scrapings or other samples to visualize dark-pigmented, septate hyphae
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Slow to moderate growth at 25-30°C, forming colonies with a dark green to black color and a velvety or powdery texture
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic conidiophores and conidia
  - Conidia are typically one- or two-celled and form branching chains
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification

Fonsecaea spp.

Etiology: Fonsecaea is a genus of dematiaceous molds that is a common cause of chromoblastomycosis
Epidemiology
- Found primarily in tropical and subtropical regions
- Infections are acquired through traumatic inoculation of the skin with contaminated soil or vegetation
- Risk factors:
  - Exposure to outdoor environments in tropical or subtropical regions
  - Trauma
  - Poor hygiene
Transmission
- Traumatic inoculation of the skin
Disease States
- Chromoblastomycosis: A chronic, localized infection of the skin and subcutaneous tissue, characterized by warty nodules, often on the lower extremities. The nodules can develop into large, cauliflower-like masses
Laboratory Diagnosis
- Specimen Collection
  - Skin Biopsy: From warty nodules
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of tissue samples to visualize dark-pigmented, septate hyphae and sclerotic bodies (copper pennies)
  - Sclerotic bodies are characteristic of chromoblastomycosis
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Slow growth at 25-30°C, forming colonies with a dark green to black color and a velvety or waxy texture
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic conidiophores and conidia
  - Fonsecaea species exhibit multiple types of conidiation, including Cladosporium-type, Phialophora-type, and Rhinocladiella-type
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification

Laboratory Procedures: Key Steps

Specimen Collection
- Collect appropriate specimens based on the suspected site of infection (e.g., skin scrapings, tissue biopsy, sinus aspirate)
Direct Microscopic Examination
- Prepare a KOH mount or Gram stain of tissue samples
- Examine under a microscope for dark-pigmented, septate hyphae
Culture
- Inoculate the specimen onto Sabouraud Dextrose Agar (SDA) or other fungal media
- Incubate at 25-30°C
- Examine regularly for growth
Identification
- Identify fungal isolates based on macroscopic (colony morphology) and microscopic characteristics (e.g., conidiophore and conidia morphology)
- Molecular methods (PCR, sequencing) are increasingly used for rapid and accurate identification

Key Takeaways

Dematiaceous molds are a diverse group of fungi characterized by dark-pigmented hyphae and conidia
Alternaria, Cladosporium, and Fonsecaea are common genera causing human infections
Infections are acquired through inhalation of spores or direct inoculation into the skin
Laboratory diagnosis involves direct microscopic examination and culture
Accurate identification of dematiaceous molds is essential for guiding appropriate treatment

Key Terms

Dematiaceous: Dark-pigmented
Hyphae: The thread-like, branching filaments that form the mycelium of a fungus
Septate Hyphae: Hyphae that are divided into compartments by cross-walls (septa)
Conidia: Asexual spores produced by fungi
Conidiophore: A specialized hyphal structure that produces conidia
Chromoblastomycosis: A chronic, localized infection of the skin and subcutaneous tissue, characterized by warty nodules
Sclerotic Bodies: Copper-colored, round structures found in tissue samples from patients with chromoblastomycosis
KOH Preparation: A microscopic technique where a sample is mixed with potassium hydroxide (KOH) to dissolve keratin, making fungal elements more visible
Sabouraud Dextrose Agar (SDA): A common fungal culture medium
Mould-Specific Identification: Refers to laboratory techniques, including microscopic examination and molecular assays, used to accurately identify specific mold species in clinical samples, aiding in diagnosis and treatment of infections caused by these fungi
Panfungal PCR: is a molecular diagnostic technique that utilizes polymerase chain reaction (PCR) to amplify and detect conserved DNA sequences present in a wide range of fungi, allowing for the identification of fungal infections, even when traditional culture methods are negative or impractical

Hyaline Molds

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