Mycology: Overview

Fungi belong to the eukaryote domain and, like plants, have cell walls and vacuoles, exhibit cytoplasmic streaming, and are immobile. Almost all fungi, however, have cell walls composed of chitin and not cellulose. Fungi do not carry out photosynthesis but obtain their substrates for metabolism as saprophytes (obtain their food from dead matter). Mycosis is an infection caused by fungi.

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Growth Forms and Reproduction

Growth forms

  • Multicellular forms include molds and mushrooms:
    • Mostly composed of thread-like hyphae:
      • Septated: Cell walls subdivide the hyphae.
      • Coenocytic: Hyphae lack cell walls.
    • Familiar “mushrooms” are reproductive organs (thallus) of fungi.
    • Tubular network systems in which hyphae form branching nets (mycelium)
  • Unicellular forms include yeasts:
    • Yeasts do not have hyphae or mycelium.
    • Some species form pseudo-multicellular structures (budding yeasts).
  • Dimorphic fungi:
    • Can exist as unicellular or multicellular forms
    • Environmental factors (temperature, humidity, presence of nutrients) can trigger changes.

Structure

  • Fungal cells have characteristics similar to both plant and animal cells:
    • Characteristics that are common with animal cells:
      • Presence of a membrane-bound nucleus
      • Organelles such as mitochondria
      • Lack of chloroplasts
    • Characteristics that are common with plant cells:
      • Presence of a cell wall
      • Contain vacuoles
  • Fungi have unique properties that distinguish them from animals and plants:
    • Cell walls contain chitin, unlike plant cell walls that contain cellulose.
    • Cell membranes contain ergosterol, unlike animal cell membranes that contain cholesterol.

Reproduction

Reproduction is either sexual or asexual.

  • Types of asexual reproduction: 
    • Binary fission
    • Breakdown of hyphae, dispersal, and regrowth
    • Budding (yeasts)
    • Formation of conidia (containing asexual mitotic spores)
  • Sexual reproduction:
    • Merging of 2 haploid cells:
      • Self-fertilization: 2 cells from the same organism
      • Cross-fertilization: cells from different organisms
    • The emerging diploid zygote undergoes meiosis to form haploid spores.
    • Spores germinate to create more haploid mycelia.
Budding in fungus

Budding in fungus:
Budding yeasts divide, asymmetrically. There are haploid and diploid states (2 mating types, a and α). Each mating type secretes its own type of pheromone.
1: Budding: Mitotic cell division can occur in the haploid and diploid states, resulting in genetically identical daughter cells.
2: Mating: Each mating type secretes its own type of pheromone, initiating the mating process. This results in a diploid cell.
3: Sporulation: Diploid cells can undergo meiosis, which results in spore formation. These spores can germinate into haploid cells.

Image: “Yeast lifecycle” by Masur. License: Public Domain

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Toxic Products of Fungi

Table: Toxic products of fungi
ToxinFungusEffect
Aflatoxin
  • Aspergillus flavus
  • A. parasiticus
Highly carcinogenic and often the cause of food poisoning (traces on nuts, grain, spices)
AmanitinAmanita phalloides (death cap mushroom)Inhibition of RNA polymerase II, lethal even in small doses
MuscarineA. muscaria (toadstool or fly agaric mushroom)Impacts the parasympathetic regulation of the nervous system
ErgotamineErgot fungus (Claviceps purpurea)Impacts the autonomic nervous system, causes hallucinations, and affects uterine contractions
Cyclosporine A
  • Cylindrocarpon
  • Tolypocladium
Immunosuppressant (clinical use: after organ transplantation)

Pathogenic Effect of Fungi

  • Mycoses: infectious diseases caused by fungi:
    • Usually do not pose problems in healthy individuals
    • May cause opportunistic infections in individuals who are immunocompromised
  • Classes:
    • Superficial mycoses:
      • Oropharynx: commonly seen in infants, elderly, or immunocompromised individuals
      • Anogenital area: Vaginal yeast infections are commonly seen even in healthy individuals.
      • Skin: keratinophilic fungi (also known as dermatophytes) that affect human skin, hair, and nails
    • Systemic mycoses: 
      • Develop when fungal spores are inhaled or introduced into the bloodstream
      • Can manifest in different inner organs, resulting in severe lethal infections
      • Patients who are HIV positive or immunosuppressed are susceptible to systemic mycoses.
  • Examples:
    • Thrush: infection of the oral mucosa; Candida species
    • Onychomycosis: infection of the nail; Trichophyton rubrum
    • Dermatophytosis: infection of the skin (tinea pedis, corporis, or cruris)
      • Microsporum canis, M. audouinii, M. gypseum
      • Epidermophyton floccosum
      • Trichophyton tonsurans, T. mentagrophytes, T. verrucosum, T. schoenlenii
    • Coccidioidomycosis: a pulmonary or disseminated fungal infection
      • Coccidioides immitis 
      • C. posadasii
Tinea pedis

Tinea pedis is also known as athlete’s foot.

Image: “Tinea pedis interdigitalis” by Falloonb. License: Public Domain

Synthesis of Antimicrobials

Some fungi are capable of producing substances that are effective as antimicrobials:

  • Penicillin from Penicillium notatum
  • Cephalosporin from Acremonium
  • Streptomycin from Streptomyces griseus
  • Griseofulvin from P. griseofulvum
  • 50 of approximately 2,000 substances characterized as antibiotics are used as chemotherapeutics.

Clinical Relevance

Opportunistic fungal infections

Many fungi are opportunists and are especially pathogenic in patients who are immunocompromised. Opportunistic systemic fungal infections (mycoses) include candidiasis, aspergillosis, mucormycosis, and fusariosis, and typically manifest with rapidly progressive pneumonia or fungemia.

Primary fungal infections

Caused by inhalation of fungal spores, which results in pneumonia. Different infections have specific geographic distribution: 

  • Coccidioidomycosis: Southwestern US, Washington, Northern Mexico, and Central and South America
  • Histoplasmosis: Eastern and Midwestern US and parts of Central and South America, Africa, Asia, and Australia
  • Blastomycosis: confined to North America and Africa
  • Paracoccidioidomycosis: South America

References

  1. Centers for Disease Control and Prevention. (2019). Types of Fungal Diseases. http://www.cdc.gov/fungal/diseases/index.html
  2. Blackwell, M. (2011). The fungi: 1, 2, 3 … 5.1 million species? American Journal of Botany. 98(3), 426–438. https://pubmed.ncbi.nlm.nih.gov/21613136/
  3. Centers for Disease Control and Prevention. (2015). Candidiasis. http://www.cdc.gov/fungal/diseases/candidiasis/index.html
  4. Centers for Disease Control and Prevention. (2014). Vaginal Candidiasis. http://www.cdc.gov/fungal/diseases/candidiasis/genital/index.html
  5. Centers for Disease Control and Prevention. (2015). Invasive Candidiasis. http://www.cdc.gov/fungal/diseases/candidiasis/invasive/index.html
  6. Delaloye, J., Thierry, C. (2014). Invasive candidiasis as a cause of sepsis in the critically ill patient. Virulence. 5(1), 161–169. https://pubmed.ncbi.nlm.nih.gov/24157707/
  7. Centers for Disease Control and Prevention. (2015). Ringworm. http://www.cdc.gov/fungal/diseases/ringworm/index.html
  8. Centers for Disease Control and Prevention. (2014). Sporotrichosis. http://www.cdc.gov/fungal/diseases/sporotrichosis/index.html
  9. Centers for Disease Control and Prevention. (2015). Aspergillosis. http://www.cdc.gov/fungal/diseases/aspergillosis/index.html
  10. Centers for Disease Control and Prevention. (2015). Blastomycosis. http://www.cdc.gov/fungal/diseases/blastomycosis/index.html
  11. van de Peppel, R.J., et al. (2018). The burden of invasive aspergillosis in patients with haematological malignancy: A meta-analysis and systematic review. The Journal of Infection. 76(6), 550–562. https://pubmed.ncbi.nlm.nih.gov/29727605/
  12. Fernández-Cruz, A., et al. (2018). Bronchoalveolar lavage fluid cytology in culture-documented invasive pulmonary aspergillosis in patients with hematologic diseases: Analysis of 67 episodes. Journal of Clinical Microbiology. 56(10), e00962–e00918. https://pubmed.ncbi.nlm.nih.gov/30021823/
  13. Mohedano Del Pozo, R.B., et al. (2018). Diagnosis of invasive fungal disease in hospitalized patients with chronic obstructive pulmonary disease. Revista Iberoamericana de Micología. 35(3), 117–122. https://pubmed.ncbi.nlm.nih.gov/30078525/
  14. Alanio, A., Bretagne, S. (2017). Challenges in microbiological diagnosis of invasive Aspergillus infections. F1000Res. 6(F1000) Faculty Rev-157. https://pubmed.ncbi.nlm.nih.gov/28299183/
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