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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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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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 Organelles A cell is a complex unit that performs several complex functions. An organelle is a specialized subunit within a cell that fulfills a specific role or function. Organelles are enclosed within their own lipid bilayers or are unbound by membranes. The Cell: 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 Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week: 2 cells from the same organism
      • Cross- fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week: cells from different organisms
    • The emerging diploid zygote undergoes meiosis Meiosis The creation of eukaryotic gametes involves a DNA replication phase followed by 2 cellular division stages: meiosis I and meiosis II. Meiosis I separates homologous chromosomes into separate cells (1n, 2c), while meiosis II separates sister chromatids into gametes (1n, 1c). 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 Meiosis The creation of eukaryotic gametes involves a DNA replication phase followed by 2 cellular division stages: meiosis I and meiosis II. Meiosis I separates homologous chromosomes into separate cells (1n, 2c), while meiosis II separates sister chromatids into gametes (1n, 1c). 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
Toxin Fungus Effect
Aflatoxin
  • Aspergillus flavus
  • A. parasiticus
Highly carcinogenic and often the cause of food poisoning (traces on nuts, grain, spices)
Amanitin Amanita phalloides (death cap mushroom) Inhibition of RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure polymerase II, lethal even in small doses
Muscarine A. muscaria (toadstool or fly agaric mushroom) Impacts the parasympathetic regulation of the nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System
Ergotamine Ergot fungus (Claviceps purpurea) Impacts the autonomic nervous system Autonomic nervous system The ANS is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ANS consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System, causes hallucinations, and affects uterine contractions
Cyclosporine A
  • Cylindrocarpon
  • Tolypocladium
Immunosuppressant (clinical use: after organ transplantation Organ Transplantation Transplantation is a procedure that involves the removal of an organ or living tissue and placing it into a different part of the body or into a different person. Organ transplantations have become the therapeutic option of choice for many individuals with end-stage organ failure. 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 Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin: keratinophilic fungi (also known as dermatophytes Dermatophytes Tinea infections are a group of diseases caused by fungi infecting keratinized tissue (hair, nails, and skin). These infections are termed dermatomycoses and are caused by the dermatophyte fungi. There are approximately 40 dermatophyte fungi that are part of 3 genera, including Trichophyton, Epidermophyton, and Microsporum. These infections can affect any part of the body but occur most often in warm, moist regions like the groin and the feet. Dermatophytes/Tinea Infections) 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 Candida Candida is a genus of dimorphic, opportunistic fungi. Candida albicans is part of the normal human flora and is the most common cause of candidiasis. The clinical presentation varies and can include localized mucocutaneous infections (e.g., oropharyngeal, esophageal, intertriginous, and vulvovaginal candidiasis) and invasive disease (e.g., candidemia, intraabdominal abscess, pericarditis, and meningitis). Candida/Candidiasis 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 Coccidioidomycosis Coccidioidomycosis, commonly known as San Joaquin Valley fever, is a fungal disease caused by Coccidioides immitis or Coccidioides posadasii. When Coccidioides spores are inhaled, they transform into spherules that result in infection. Coccidioidomycosis is also a common cause of community-acquired pneumonia and can cause severe disease in the immunocompromised. Coccidioides/Coccidioidomycosis: a pulmonary or disseminated fungal infection
      • Coccidioides Coccidioides Coccidioidomycosis, commonly known as San Joaquin Valley fever, is a fungal disease caused by Coccidioides immitis or Coccidioides posadasii. When Coccidioides spores are inhaled, they transform into spherules that result in infection. Coccidioidomycosis is also a common cause of community-acquired pneumonia and can cause severe disease in the immunocompromised. Coccidioides/Coccidioidomycosis 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 Griseofulvin In addition to the 3 other major classes of antifungal agents (azoles, polyenes, and echinocandins), several other clinically important antifungal agents are used, including flucytosine, griseofulvin, and terbinafine. Griseofulvin acts within the stratum corneum of the skin and are used to treat dermatophyte infections of the skin, hair, and nails. Flucytosine, Griseofulvin, and Terbinafine 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 Candidiasis Candida is a genus of dimorphic, opportunistic fungi. Candida albicans is part of the normal human flora and is the most common cause of candidiasis. The clinical presentation varies and can include localized mucocutaneous infections (e.g., oropharyngeal, esophageal, intertriginous, and vulvovaginal candidiasis) and invasive disease (e.g., candidemia, intraabdominal abscess, pericarditis, and meningitis). Candida/Candidiasis, aspergillosis Aspergillosis Aspergillosis is an opportunistic fungal infection caused by Aspergillus species, which are common spore-forming molds found in our environment. As Aspergillus species are opportunistic, they cause disease primarily in patients who are immunocompromised. The organs that are most commonly involved are the lungs and sinuses. Aspergillus/Aspergillosis, mucormycosis Mucormycosis Mucormycosis is an angioinvasive fungal infection caused by multiple fungi within the order, Mucorales. The fungi are ubiquitous in the environment, but mucormycosis is very rare and almost always occurs in patients who are immunocompromised. Inhalation of fungal spores can cause rhinocerebral or pulmonary mucormycosis, direct inoculation can cause cutaneous mucormycosis, and ingestion can cause gastrointestinal mucormycosis. Mucorales/Mucormycosis, and fusariosis, and typically manifest with rapidly progressive pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. Pneumonia or fungemia.

Primary fungal infections

Caused by inhalation of fungal spores, which results in pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. Pneumonia. Different infections have specific geographic distribution: 

  • Coccidioidomycosis Coccidioidomycosis Coccidioidomycosis, commonly known as San Joaquin Valley fever, is a fungal disease caused by Coccidioides immitis or Coccidioides posadasii. When Coccidioides spores are inhaled, they transform into spherules that result in infection. Coccidioidomycosis is also a common cause of community-acquired pneumonia and can cause severe disease in the immunocompromised. Coccidioides/Coccidioidomycosis: Southwestern US, Washington, Northern Mexico, and Central and South America
  • Histoplasmosis Histoplasmosis Histoplasmosis is an infection caused by Histoplasma capsulatum, a dimorphic fungus. Transmission is through inhalation, and exposure to soils containing bird or bat droppings increases the risk of infection. Most infections are asymptomatic; however, immunocompromised individuals generally develop acute pulmonary infection, chronic infection, or even disseminated disease. Histoplasma/Histoplasmosis: Eastern and Midwestern US and parts of Central and South America, Africa, Asia, and Australia
  • Blastomycosis Blastomycosis Blastomycosis is an infection caused by inhalation of the spores of the fungus, Blastomyces. Blastomyces species thrive in moist soil and decaying material and are common in the Ohio and Mississippi River valleys and the Great Lakes regions of the United States and Canada. Although most patients are asymptomatic, some can develop pneumonia. Blastomyces/Blastomycosis: confined to North America and Africa
  • Paracoccidioidomycosis Paracoccidioidomycosis Paracoccidioidomycosis (PCM) is an endemic fungal infection caused by Paracoccidioides brasiliensis and P. lutzii. The fungus is geographically distributed across Mexico and South and Central America. Transmission is by inhalation, and most infections are asymptomatic. Paracoccidioides/Paracoccidioidomycosis: South America

References

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  4. Centers for Disease Control and Prevention. (2014). Vaginal Candidiasis. http://www.cdc.gov/fungal/diseases/candidiasis/genital/index.html
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