Hyaline Molds

These molds can cause a wide range of infections, from relatively mild to life-threatening, especially in immunocompromised individuals. We’ll cover the key genera – Aspergillus, Fusarium, Penicillium, and the Scedosporium apiospermum complex – along with their disease states, transmission, and lab identification methods

Hyaline Molds

Definition: Hyaline molds are a diverse group of filamentous fungi characterized by hyaline (clear or colorless) hyphae
Ubiquitous: They are found ubiquitously in the environment, including soil, decaying vegetation, and indoor air
Opportunistic Pathogens: Many hyaline molds are opportunistic pathogens, meaning they typically cause infections in individuals with weakened immune systems
Key Genera: The most common genera causing human infections are Aspergillus, Fusarium, Penicillium, and Scedosporium
Conidia: They reproduce asexually by producing conidia (spores) on specialized structures called conidiophores
Disease: The specific diseases caused by hyaline molds vary depending on the genus and the immune status of the host

Aspergillus spp.

Etiology: Aspergillus is a ubiquitous genus of filamentous fungi that can cause a variety of diseases, collectively known as aspergillosis
Epidemiology
- Found worldwide in soil, decaying vegetation, and indoor air
- Infections are acquired through inhalation of conidia
- Risk factors:
  - Neutropenia (low white blood cell count)
  - Corticosteroid use
  - Hematopoietic stem cell transplantation
  - Solid organ transplantation
  - Chronic granulomatous disease
  - Cystic fibrosis
  - Lung disease (e.g., COPD, asthma)
Transmission
- Inhalation of conidia from the environment
- Not typically transmitted from person to person
Disease States
- Allergic Bronchopulmonary Aspergillosis (ABPA): An allergic reaction to Aspergillus in the lungs, common in patients with asthma or cystic fibrosis. Symptoms include wheezing, coughing, and shortness of breath
- Aspergilloma: A “fungus ball” that develops in a pre-existing lung cavity (e.g., from tuberculosis). Often asymptomatic, but can cause coughing up blood (hemoptysis)
- Chronic Pulmonary Aspergillosis (CPA): A slowly progressive lung infection characterized by cavities, nodules, and fibrosis
- Invasive Aspergillosis (IA): A severe, life-threatening infection that occurs primarily in immunocompromised individuals. Can affect the lungs, brain, heart, and other organs
- Cutaneous Aspergillosis: Infection of the skin, often at the site of trauma or catheter insertion
- Aspergillus Sinusitis: Infection of the sinuses
- Aspergillus Endocarditis: Infection of the heart valves
Common Species
- Aspergillus fumigatus: The most common cause of invasive aspergillosis
- Aspergillus flavus: Can produce aflatoxins, which are carcinogenic
- Aspergillus niger: A common cause of otomycosis (ear infection) and aspergilloma
- Aspergillus terreus: Often resistant to amphotericin B
Laboratory Diagnosis
- Specimen Collection
  - Respiratory: Sputum, bronchoalveolar lavage (BAL), lung biopsy
  - Blood: For disseminated infection
  - Tissue Biopsy: From affected organs
  - Sinus Aspirate: For suspected sinusitis
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of tissue samples to visualize septate hyphae with dichotomous branching (branching at a 45-degree angle)
  - GMS (Gomori Methenamine Silver) or PAS (Periodic Acid-Schiff) Stain: Of tissue biopsies to highlight fungal elements
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Rapid growth at 25-30°C, forming colonies with a velvety or granular texture and various colors depending on the species
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic conidiophores and conidia
  - Aspergillus fumigatus: Conidiophore with a flask-shaped vesicle and chains of conidia (phialides) covering the upper two-thirds of the vesicle
  - Aspergillus flavus: Conidiophore with a round vesicle and loosely arranged phialides
  - Aspergillus niger: Conidiophore with a round vesicle and dark brown to black conidia
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification
- Antigen Detection
  - Galactomannan Assay: Detects galactomannan, a polysaccharide component of the Aspergillus cell wall, in serum or BAL fluid. Used to diagnose invasive aspergillosis
- Antibody Detection
  - Aspergillus Antibody Test: Detects antibodies against Aspergillus in serum. Used to diagnose allergic bronchopulmonary aspergillosis (ABPA)
- Beta-D-Glucan Assay
  - Detects beta-D-glucan, a component of the cell wall of many fungi, in serum. Can be used as a broad marker for invasive fungal infections

Fusarium spp.

Etiology: Fusarium is a genus of filamentous fungi that can cause a variety of infections, including keratitis, onychomycosis, and disseminated infections
Epidemiology
- Found worldwide in soil, plants, and water
- Infections are acquired through direct inoculation into the skin or eyes, or through inhalation of conidia
- Risk factors:
  - Contact lens wear (for keratitis)
  - Trauma
  - Burns
  - Immunosuppression
Transmission
- Direct inoculation into the skin or eyes
- Inhalation of conidia
Disease States
- Fusarium Keratitis: Infection of the cornea, often associated with contact lens wear. Symptoms include eye pain, redness, blurred vision, and sensitivity to light
- Onychomycosis: Infection of the nails
- Disseminated Fusarium Infection: A severe, life-threatening infection that occurs primarily in immunocompromised individuals. Can affect the skin, lungs, and bloodstream
Laboratory Diagnosis
- Specimen Collection
  - Corneal Scrapings: For suspected keratitis
  - Nail Clippings: For suspected onychomycosis
  - Blood: For disseminated infection
  - Tissue Biopsy: From affected organs
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of corneal scrapings or nail clippings to visualize septate hyphae
  - GMS or PAS Stain: Of tissue biopsies to highlight fungal elements
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Rapid growth at 25-30°C, forming colonies with a cottony or fluffy texture and various colors depending on the species (e.g., white, pink, purple)
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic macroconidia (sickle-shaped or canoe-shaped) and microconidia
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification

Penicillium spp.

Etiology: Penicillium is a genus of filamentous fungi that is best known for producing penicillin, but some species can also cause human infections
Epidemiology
- Found worldwide in soil, air, and decaying organic matter
- Infections are rare and typically occur in immunocompromised individuals
- Risk factors:
  - Immunosuppression
  - Prolonged antibiotic use
  - Indwelling catheters
- Transmission is primarily through inhalation of conidia
Transmission
- Inhalation of conidia
- Direct inoculation into the skin
Disease States
- Penicilliosis: A disseminated infection caused by Penicillium marneffei, primarily in Southeast Asia. Symptoms include fever, skin lesions, lymphadenopathy, and hepatosplenomegaly
- Keratitis: Infection of the cornea
- Onychomycosis: Infection of the nails
- Sinusitis: Infection of the sinuses
- Endocarditis: Infection of the heart valves
Laboratory Diagnosis
- Specimen Collection
  - Skin Biopsy: From skin lesions
  - Blood: For disseminated infection
  - Bone Marrow: For disseminated infection
  - Tissue Biopsy: From affected organs
  - Corneal Scrapings: For suspected keratitis
  - Nail Clippings: For suspected onychomycosis
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of corneal scrapings or nail clippings to visualize septate hyphae
  - GMS or PAS Stain: Of tissue biopsies to highlight fungal elements
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Rapid growth at 25-30°C, forming colonies with a velvety or powdery texture and various colors depending on the species (e.g., green, blue, white)
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic conidiophores (brush-like) and conidia
  - Penicillium marneffei: Produces a red pigment that diffuses into the agar
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification

Scedosporium apiospermum Complex

Etiology: The Scedosporium apiospermum complex includes several closely related species of filamentous fungi that can cause a variety of infections, including mycetoma, sinusitis, and disseminated infections
Epidemiology
- Found worldwide in soil, sewage, and polluted water
- Infections are acquired through inhalation of conidia or direct inoculation into the skin
- Risk factors:
  - Near-drowning events
  - Trauma
  - Immunosuppression
Transmission
- Inhalation of conidia
- Direct inoculation into the skin
Disease States
- Mycetoma: A chronic, localized infection of the skin and subcutaneous tissue, characterized by swelling, draining sinuses, and granules
- Sinusitis: Infection of the sinuses
- Pneumonia: Infection of the lungs
- Disseminated Infection: A severe, life-threatening infection that occurs primarily in immunocompromised individuals. Can affect the brain, heart, and other organs
Laboratory Diagnosis
- Specimen Collection
  - Tissue Biopsy: From mycetoma or other affected sites
  - Sinus Aspirate: For suspected sinusitis
  - Sputum or BAL: For suspected pneumonia
  - Blood: For disseminated infection
  - Tissue Biopsy: From affected organs
- Direct Microscopic Examination
  - KOH Preparation or Gram Stain: Of tissue samples to visualize septate hyphae
  - GMS or PAS Stain: Of tissue biopsies to highlight fungal elements
- Culture
  - Inoculation onto Sabouraud Dextrose Agar (SDA) or other fungal media
  - Rapid growth at 25-30°C, forming colonies with a cottony or velvety texture and a gray to brown color
- Identification
  - Colony Morphology: Observation of colony color, texture, and growth rate
  - Microscopic Examination: Identification of characteristic conidia (annelloconidia)
  - MALDI-TOF MS: Rapid and accurate identification
  - Molecular Methods: PCR and sequencing for definitive identification
- Antifungal Susceptibility Testing
  - Scedosporium species are often resistant to many antifungal drugs, so susceptibility testing is important for guiding therapy

Laboratory Procedures: Key Steps

Specimen Collection
- Collect appropriate specimens based on the suspected site of infection (e.g., tissue biopsy, BAL, blood)
Direct Microscopic Examination
- Prepare a KOH mount or Gram stain of tissue samples
- Examine under a microscope for 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
Antifungal Susceptibility Testing
- Perform antifungal susceptibility testing on clinically significant isolates to guide therapy

Key Takeaways

Hyaline molds are a diverse group of fungi that can cause a variety of infections, ranging from allergic reactions to life-threatening invasive diseases
Aspergillus, Fusarium, Penicillium, and Scedosporium are the most common genera causing human infections
Laboratory diagnosis involves direct microscopic examination, culture, and molecular methods
Accurate identification of hyaline molds is essential for guiding appropriate treatment
Antifungal susceptibility testing is important for managing infections caused by resistant strains

Key Terms

Hyaline: Clear or colorless
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
Dichotomous Branching: Branching that occurs at a 45-degree angle, characteristic of Aspergillus hyphae
Galactomannan: A polysaccharide component of the cell wall of Aspergillus species. Galactomannan antigen detection is used to diagnose invasive aspergillosis
Beta-D-Glucan: A polysaccharide found in the cell walls of many fungi. Beta-D-glucan detection is used as a broad marker for invasive fungal infections
Annelloconidia: A type of conidia produced by Scedosporium species
Mycetoma: A chronic, localized infection of the skin and subcutaneous tissue, characterized by swelling, draining sinuses, and granules
Keratitis: Infection of the cornea
Onychomycosis: Infection of the nails
Allergic Bronchopulmonary Aspergillosis (ABPA): An allergic reaction to Aspergillus in the lungs
Aspergilloma: A “fungus ball” that develops in a pre-existing lung cavity
Invasive Aspergillosis (IA): A severe, life-threatening infection that occurs primarily in immunocompromised individuals
Voriconazole: A triazole antifungal drug commonly used to treat aspergillosis and other fungal infections
Posaconazole: A triazole antifungal drug used to treat aspergillosis and other fungal infections
Isavuconazole: A triazole antifungal drug used to treat aspergillosis and other fungal infections
Amphotericin B: A polyene antifungal drug used to treat severe fungal infections
Echinocandins: A class of antifungal drugs that inhibit the synthesis of beta-glucan in fungal cell walls

Mucorales

Dematiaceous Molds