General mycology

Fungi are widespread in the environment and many species colonize the human body without evidence of infection. Mycoses are infections caused by fungi such as dermatophytes (e.g., Trichophyton), yeasts (e.g., Candida), or molds (e.g., Aspergillus).

See also “Overview of fungal infections,” “Candidiasis,” “Aspergillosis,” and “Dermatophyte infections.”

Overview of mycological terminology ^[1]

• Hyphae: tubular, branching filaments of fungal cells, with or without septae
• Septa: hyphal cell wall divisions, typically porous
• Pseudohyphae
  • A hypha-like filament formed by a chain of budding yeast cells that have not become detached from each other
  • Pseudohyphae can be identified by the presence of constrictions at the site of origin.
• Mycelium: a haploid and multicellular network of hyphae forming a thread-like structure
• Pseudomycelium: mycelium-like mass of pseudohyphae
• Thallus: the vegetative body of a fungus
• Sporangia: a spore-forming structure
• Spores
  • Haploid, reproductive particles that can be formed on reproductive hyphae or mycelium
  • May result from asexual (e.g., sporangiospores) or sexual reproduction
• Sporangiophore: modified hyphae bearing sporangia
• Conidia
  • Asexual spore of a fungus
  • May bud in clusters of chains
  • Produced by most pathogenic fungi
• Conidiophore: simple or branched hyphae on which conidia are produced
• Germ tube
  • An outgrowth produced by spores during germination
  • No constrictions at the site of origin
• Anamorph: asexual reproductive state of a fungus
• Teleomorph: sexual reproductive state of a fungus
• Conidium: an asexual fungal spore formed from a vegetative yeast, hyphal cell, or a specialized conidiogenous cell
  • Microconidium: a small asexual fungal spore
  • Macroconidium: a large asexual fungal spore

Fungi structure and morphology ^[2]

Fungi vary widely in size and shape, from unicellular to multicellular forms. Microscopic fungi are either molds or yeasts or both.

• General characteristics of fungi
  • Fungi are eukaryotes.
  • Most are obligate or facultative aerobes.
  • Can be unicellular, multicellular, or dimorphic
  • Divide asexually, sexually, or both
  • Fungi are chemotrophic organisms, i.e., they secrete enzymes to degrade organic substrates.
• Structure of fungi
  • Fungal cell wall: composed of chitin, glucans, and mannans
  • Fungal cell membrane: contains ergosterol (analogous to cholesterol in humans)
    • Cell membrane component unique to fungal species
    • Key enzymes in its synthetic pathway include squalene epoxidase and 14–α–demethylase, which converts lanosterol to ergosterol.
• Morphology of fungi
  • Monomorphic fungi: fungi that exist exclusively as either yeast or mold
    • Yeasts: unicellular fungi, occasionally organize into hyphae and pseudohyphae
    • Molds: multicellular fungi, organized into hyphae
    • For clinical significance and further discussion, see “Yeasts” and “Molds” below.
  • Dimorphic fungi: fungi that can exist as both mold or yeast, depending on temperature
    • Molds typically grow at approx. 20°C and yeasts grow at 37 °C.
    • Examples: Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, and Sporothrix schenckii
    • Most commonly cause subacute pulmonary infections

Fungi represent a kingdom separate from plants and animals. They extract energy (e.g., sugar and proteins) from living or dead organic matter.

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

Fungi replication ^[1][3][4]

Overview

Fungi reproduce through sexually and/or asexually produced spores.

• Asexual reproduction
  • Occurs through mitotic division
  • Produces unicellular spores that are genetically identical to the parent body
• Sexual reproduction
  • Occurs through meiotic division
  • Produces zygospores, which result from the union of two compatible nuclei and are genetically different from the parent bodies
  • Introduces genetic diversity

Asexual reproduction of fungi

• Mechanisms
  • Asexual spore production (most common mode of reproduction)
    • Spores may be released outside or within a special reproductive structure, where they grow independently.
    • A haploid cell is produced via mitosis and released from the parent body (e.g., hypha, sporogenous cell)
  • Fungal fragmentation: a portion of the mycelium splits from the body of the fungus, with the resulting fragment being capable of replication
  • Binary fission: a cell undergoes nuclear division and splits into two daughter cells capable of replicating
  • Budding
    • An outgrowth, bud, or appendage develops on the surface of the cell or the hypha, with its cytoplasm being continuous with that of the parent cell.
    • The nucleus of the parent cell divides and one daughter nucleus migrates into the bud.
    • This fragment eventually detaches from the parent cell after the nucleus divides via mitosis and is capable of replicating using the same process.
    • Yeasts (e.g., Candida spp.) mainly reproduce using this mechanism.
• Types of asexual spores
  • Conidiospore: a single-cell, bicellular, or multicellular structure that arises on the tip or side of a hyphal structure called a “conidiophore” (e.g., Aspergillus)
  • Sporangiospore: produced in a sac-like structure called sporangia (e.g., Rhizopus)
  • Arthrospore: formed by the fragmentation of a disjunctor cell or splitting of a double septum (e.g., Trichosporidium, Coccididious immitis)
  • Chlamydospore: a thick-walled, single-cell spore in or on hyphae (e.g., Candida spp.)
  • Blastospore: a budding spore usually formed at the terminal end of a hypha (e.g., Candida albicans, Trichosporon spp., Paracoccidioides spp.)

Sexual reproduction of fungi

• Stages: plasmogamy, karyogamy, and meiosis
  • Union of two haploid hyphae → fusion of the cytoplasm/protoplasm (plasmogamy) → fusion of the nuclei (karyogamy) → a diploid zygote
  • Under appropriate environmental stimuli (e.g., light, temperature, nutrients), the zygote undergoes meiosis → production of sexual spores → germination
• Main methods of plasmogamy
  • Gametangium
    • An organ or cell in which gametes are produced
    • The fusion of the entire contents of two contacting gametangia takes place via a pore-like structure or by direct fusion.
  • Somatogamy: somatic hyphae take over the sexual function, come into contact, fuse, and exchange nuclei
  • Spermatization: fungi male fungal structures called spermatia become attached to the female receptive hyphae or gametangia
• Types of sexual spores
  • Ascospores: produced in a sac-like structure called an “ascus” (e.g., Blastomyces, Histoplasma, Aspergillus)
  • Basidiospore: produced in a club-shaped structure called a “basidium” (e.g., Cryptococcus, Malassezia, Trichosporon)
  • Zygospore: formed when two sexually compatible hyphae or gametangia fuse (e.g., Rhizopus)
  • Oospores: a thick-walled spore, formed when a female gamete is fertilized by a male gamete nucleus. I.e., occurs solely in clinically nonrelevant fungi, e.g., Pythium, Phytophthora.

Sexual systems ^[5]

• Overview
  • There are two main modes of reproduction: homothallism and heterothallism
  • Both modes share key reproductive features (e.g., ploidy changes, meiosis) but differ in features involving aspects of cell or hyphal fusion.
• Homothallism
  • The possession of resources to reproduce sexually in a single fungus; i.e., the thallus contains both male and female sex organs, thus enabling self-fertilization
  • Examples: Aspergillus nidulans, P. jirovecii
• Heterothallism
  • The division of sexes between fungi of a single (self-sterile) species; reproduction occurs between two separate mycelia
  • Example: Saccharomyces cerevisiae (baker's yeast)

Fungal metabolism ^[1][6]

Overview

• Fungi have a variety of metabolic pathways enabling them to digest and synthesize compounds.
• Fungi derive their energy from the breakdown of organic compounds.
• Organic compound breakdown is achieved via enzyme secretion (e.g., hydrolytic, oxidative, peroxidative)

Substances synthesized by fungi

• Primary metabolites
  • Compounds responsible growth and energy storage
  • Act as intermediates in the biosynthesis of secondary metabolites
  • Examples: pyruvate, ethanol, lactic acid
• Secondary metabolites: compounds not essential to cellular life
  • Antibiotics: e.g., penicillins by Penicillium chrysogenum, cephalosporins by Acremonium chrysogenum
  • Toxins
    • Aflatoxins by molds (e.g., Aspergillus flavus)
      • Found on nuts, seeds, and grains
      • Poisonous carcinogen associated with hepatocellular carcinoma
    • Amanitins (see “Amanita phalloides”)
    • Ergot alkaloids: ergotamine by ergot fungus (e.g., Claviceps purpurea)
      • Found on grains, typically rye
      • Causes vasoconstriction by binding to 5-HT1D serotonin receptors and alpha-adrenergic receptors

• Dermatophytes: filamentous fungi (mainly affect skin and nails)
  • Trichophyton
  • Microsporum
  • Epidermophyton species
• Yeasts: budding fungi (may affect skin, mucous membranes, and internal organs)
  • Candida species (particularly Candida albicans) cause candidiasis
  • Cryptococcus neoformans causes cryptococcosis
  • Malassezia furfur causes tinea versicolor
• Molds: Aspergillus fumigatus causes aspergillosis (may affect internal organs)
• Dimorphic fungi
  • Blastomyces dermatitidis
  • Coccidioidomycosis species
  • Histoplasma capsulatum
  • Sporothrix schenckii
  • See “Endemic fungal infections.”

• Pathogenicity
  • Dermatophytes produce keratinase, which allows them to infect the skin, hair, and nails.
  • Many dermatophytes are obligate pathogens.
• Morphology
  • Form hyphae and mycelium
  • After penetration into the skin, concentric propagation may occur around the entry site.

• Pathogenicity
  • Usually an opportunistic pathogen
  • Can infect the skin, mucous membranes, or internal organs
• Morphology
  • Demonstrate budding cells and pseudomycelium
  • Single yeast cells are 5–8 μm in size.
  • Gram positive staining

• Pathogenicity
  • Usually an opportunistic pathogen
  • Mold antigens are one of the most frequent causes of allergies.
  • Molds produce various toxins. Infection is caused by either aerogenic uptake or is foodborne (e.g., aflatoxins).
• 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.
• Pathogen: Aspergillus fumigatus
  • Ubiquitous occurrence
  • Some species produce aflatoxins
• Diseases
  • Aspergillosis
  • Allergic bronchopulmonary aspergillosis (ABPA)
  • Aspergilloma
  • Invasive pulmonary aspergillosis
  • Endocarditis
• Treatment
  • ABPA
    • Oral prednisone if severe
    • Itraconazole if recurrent
  • Pulmonary aspergilloma: lobectomy
  • Invasive aspergillosis: IV voriconazole, caspofungin, amphotericin