• Clinical science

General mycology

Summary

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.

Basics of mycology

Structure and characteristics of fungi

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 → evaluation of fungal morphology
  • Additional characterization via:
    • Culture, (e.g., Sabouraud agar)
    • Staining (e.g., silver stain)
    • Antigen detection (e.g., capsule components in serum)
    • Antibody detection plays a minor role.
  • Wood lamp examination: used to evaluate superficial fungal infections of the skin (e.g., erythrasma), which appear characteristically fluorescent under ultraviolet light
  • Latex agglutination test

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.

Fungi overview

Opportunistic fungal infections

Mycosis Etiology Clinical features Diagnosis Treatment
Aspergillosis
Candidiasis
Cryptococcosis
Pneumocystis pneumonia
  • Chest x-ray or CT: diffuse, bilateral ground-glass opacities
  • Bronchoalveolar lavage or lung biopsy (if sputum is negative): silver stain and immunofluorescence show disc-shaped yeasts

Mucormycosis

  • Pathogen: zygomycetes Mucor and Rhizopus
  • Risk factors: diabetic and immunocompromised patients
  • Patients with chronic sinus infection lasting for more than 3 months are predisposed to mucormycosis.
  • Head CT: sinusitis with orbital and intracranial involvement
  • Tissue biopsy (confirmatory): wide-angled branching of broad nonseptate hyphae

Cutaneous fungal infections

Mycosis Etiology Clinical features Diagnosis Treatment
Dermatophytes
Tinea versicolor (pityriasis versicolor)
  • Best initial: KOH showing short hyphae and spores that have a “spaghetti and meatballs” appearance

Systemic fungal infections

Mycosis Etiology Clinical features Diagnosis Treatment
Histoplasmosis
Coccidioidomycosis (valley fever)
Paracoccidioidomycosis
  • Infected patients often asymptomatic
  • Acute pneumonia
  • Painful nasal, pharyngeal, and laryngeal mucosal ulcerations
  • Lymphadenopathy (usually cervical)
  • Can disseminate → extrapulmonary manifestations (including verrucous skin lesions)
  • KOH/calcofluor staining on smears or silver/PAS-staining on tissue biopsy: budding yeast with “captain's wheel” formation
  • Cultures have a low sensitivity.

Blastomycosis

  • Pneumonia
  • Extrapulmonary: can disseminate to the skin (verrucous lesions and granulomatous nodules) and bones
  • KOH or culture (confirmatory) of sputum, urine, or body fluids showing:
    • yeast form (at body temperature or > 37°C): broad-based buds
    • Mold form (at room temperature): circular fungal cells with filamentous hyphae

Other fungal infections

Sporotrichosis (Rose gardener disease)

References:[1]

Dermatophytes

Important pathogens 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

Yeasts

Important pathogens 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

Molds

  • Pathogenicity
  • Morphology
    • Formation of monomorphic septate hyphae that branch at 45° and mycelium
    • Mold can infest food products and acquire a fuzzy appearance that is macroscopically visible.
Important pathogens Characteristics Diseases Treatment

Aspergillus fumigatus

  • Ubiquitous occurrence
  • Some species produce aflatoxins

  • 1. Bradsher RW. Mycology, Pathogenesis, and Epidemiology of Blastomycosis. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/mycology-pathogenesis-and-epidemiology-of-blastomycosis. Last updated July 26, 2017. Accessed November 16, 2018.
  • Kumar V, Abbas AK, Aster JC. Robbins & Cotran Pathologic Basis of Disease. Philadelphia, PA: Elsevier Saunders; 2014.
  • Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • Brooks G, Carroll KC, Butel J, Morse S, Mietzner TA. Jawetz Melnick & Adelbergs Medical Microbiology. New York, NY: McGraw Hill Professional; 2012.
last updated 11/29/2019
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