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General mycology

Last updated: August 31, 2021

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Mycoses are infections caused by fungi. They may be caused by dermatophytes (e.g., Trichophyton), yeast (e.g., Candida), or molds (e.g., Aspergillus). In immunocompetent individuals, mycoses usually result in local infection, which can be treated with local antifungals. Fungal infections may cause systemic infection in immunocompromised individuals (e.g., HIV-positive individuals, bone marrow transplant recipients), potentially leading to meningitis or severe sepsis.

Structure and characteristics of fungi

  • Fungi are eukaryotes.
  • Fungal cell wall and membrane: Fungi possess a cell wall and a cell membrane that contains ergosterol (analogous to cholesterol in humans).
  • Fungal appendages
    • Particularly in dermatophytes and molds
      • Hyphae: multicellular compartments, which are connected by porous septa
      • Mycelium: a network of hyphae, which are formed by asexual reproduction
    • Particularly in yeasts
      • Budding cells (blastospores): in unicellular fungi, formed by budding of daughter cells
      • Pseudomycelium (pseudohyphae): chains of budding cells, which are stretched in a hyphen-like manner but are divided by septa

Fungi are not considered plants and are nonphotosynthetic. They extract energy (e.g., sugar and proteins) from living or dead organic matter.

Azoles target the synthesis of ergosterol, the principal sterol in fungal cell membranes. They inhibit the synthesis of ergosterol from lanosterol by interfering with 14α-demethylase (cytochrome P-450 enzyme).

Substances synthesized by fungi

Detection of fungal infections

  • Clinical features and microscopy for evaluation of fungal morphology
  • Additional characterization via:
    • Culture, e.g., Sabouraud agar (a fungal growth medium that contains dextrose and peptones)
    • Staining, e.g.:
      • Silver stain
      • India ink stain: a type of negative stain with carbon that is most commonly used to identify organisms with a polysaccharide capsule such as Cryptococcus neoformans (the capsule is not penetrated by the ink and appears as a halo around the organism against a dark background)
      • Mucicarmine: a staining method used to identify the thick polysaccharide capsule of some organisms (e.g., Cryptococcus neoformans) and mucin (e.g., in gastric tumors); mucicarmine stains bright red
    • Antigen detection, e.g., capsule components in serum
    • Antibody detection plays a minor role.
  • Wood lamp examination: a diagnostic test to examine skin lesions with a lamp that emits ultraviolet light
  • Latex agglutination test

To remember the temperatures at which the different forms of dimorphic fungi exist, think of “Mold in the cold, yeast in the beast!”. Dimorphic fungi exist as molds at cooler temperatures (cold) and as yeasts at warmer temperatures (beastly heat).

Overview of the most common opportunistic fungal infections
Pathogen Risk factors Clinical features Diagnostics Treatment
Aspergillosis
Candidiasis
Cryptococcosis
  • Cryptococcus neoformans
  • AIDS
  • Exposure to pigeon droppings/soil
  • Transmission via inhalation
Pneumocystis pneumonia

Mucormycosis

  • Zygomycetes Mucor and Rhizopus (e.g., Rhizopus oryzae)
  • Imaging
    • Assess the extent of tissue damage and organ involvement
    • Head CT: sinusitis with orbital and intracranial involvement
  • Tissue biopsy (confirmatory): wide-angled branching of irregularly shaped, broad, nonseptate hyphae

Candida, Aspergillus, and Cryptococcus are opportunistic fungal pathogens with low inherent virulence. They commonly cause systemic mycoses in immunocompromised hosts but do not normally affect healthy hosts.

Overview of the most common cutaneous fungal infections
Pathogen Risk factors Clinical features Diagnostics Treatment
Dermatophytes
Tinea versicolor (pityriasis versicolor)
  • Hot or humid weather conditions
  • Best initial: KOH showing short hyphae and spores that have a “spaghetti and meatballs” appearance

Overview of the most common systemic fungal infections
Pathogen Risk factors Clinical features Diagnostics Treatment
Histoplasmosis
  • Histoplasma capsulatum
  • Endemic areas: Mississippi and the Ohio river valley
  • Exposure to bird or bat droppings in endemic areas through activities such as spelunking (cave exploration)
  • Immunosuppression (e.g., AIDS)
Coccidioidomycosis (valley fever)
  • Coccidioides immitis
  • Coccidioides posadasii
  • Travel to Southwestern United States, California
  • Soil/dust exposure in endemic areas (e.g., during windstorms, earthquakes, archeological explorations) [2]
Paracoccidioidomycosis
  • Paracoccidioides species
    • Paracoccidioides brasiliensis
    • Paracoccidioides lutzii
  • Travel to South and Central America
  • > [4]
  • KOH/calcofluor staining on smears or silver/PAS-staining on tissue biopsy
    • Budding yeast with “captain's wheel” formation
    • Fungi are identified by comparing their size to that of an RBC (fungal size > RBC)
  • Cultures have low sensitivity.

Blastomycosis

  • Blastomyces dermatitidis
  • Travel to Southeastern, Central, Eastern, and the Great Lakes region of the United States
  • KOH or culture (confirmatory) of sputum, urine, or body fluids showing:
    • Yeast form (at body temperature or > 37°C): broad-based buds
    • Fungi are identified by comparing their size to that of an RBC (fungal size ≈ RBC)
    • Mold form (at room temperature): circular fungal cells with filamentous hyphae

History of the hidden Ohio and Mississippi river valleys:” Histoplasma is hidden within macrophages and Ohio and Mississippi river valleys are the endemic regions of histoplasma.

Paracoccidiomycosis steers the ship to South and Central America at the captain's wheel: ”Paracoccidiomycosis is endemic in South and Central America and its budding yeast has a captain's wheel appearance.

The yeast form of Blastomycosis forms broad-based buds.

Unlike most other dimorphic fungi, Blastomyces can cause disseminated disease even in immunocompetent hosts.

Sporotrichosis (Rose gardener disease)

“A rose gardener plants roses in a pot while smoking a cigar:” sporotrichosis is associated with traumatic gardening injuries, treatment includes potassium iodide, and Sporothrix appears as a cigar-shaped yeast in culture.

Overview of the most important dermatophytes
Characteristics Diseases Treatment

Trichophyton species

  • Occurs worldwide
  • Partial yellow-green fluorescence under Wood lamp

Epidermophyton species

  • Occurs worldwide
  • No typical fluorescence under Wood lamp

Microsporum species

  • Occurs worldwide
  • Partial blue-green fluorescence under Wood lamp

Overview of the most important yeasts
Characteristics Diseases Treatment

Candida species

Cryptococcus neoformans

  • Humans are infected via contaminated dust particles.
  • Possesses a capsule, which can be visualized using India ink

Malassezia furfur

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  2. Thompson G, Brown J, Benedict K, Park B. Coccidioidomycosis: epidemiology. Clinical Epidemiology. 2013 : p.185. doi: 10.2147/clep.s34434 . | Open in Read by QxMD
  3. Arora NP, Taneja V, ReyesSacin C, Bhanot R, Natesan SK. Coccidioidomycosis masquerading as malignancy.. BMJ case reports. 2012; 2012 . doi: 10.1136/bcr.12.2011.5357 . | Open in Read by QxMD
  4. Paracoccidioidomycosis. https://rarediseases.org/rare-diseases/paracoccidioidomycosis/. Updated: January 1, 2009. Accessed: April 22, 2020.
  5. Blastomycosis. https://rarediseases.org/rare-diseases/blastomycosis/. Updated: January 1, 2009. Accessed: May 2, 2020.
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  7. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. McGraw-Hill Medical ; 2017
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