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).
Basics of mycology
Overview of mycological terminology 
- Hyphae: tubular, branching filaments of fungal cells, with or without septae
- Septa: hyphal cell wall divisions, typically porous
- 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
- Sporangiophore: modified hyphae bearing sporangia
- 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
- 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 
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)
Morphology of fungi
- Monomorphic fungi: fungi that exist exclusively as either yeast or mold
- Dimorphic fungi: fungi that can exist as both mold or yeast, depending on temperature
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).
Physiology of fungi
Fungi replication 
Fungi reproduce through sexually and/or asexually produced spores.
- Occurs through mitotic division
- Produces unicellular spores that are genetically identical to the parent body
- Sexual reproduction
Asexual reproduction of fungi
- Asexual spore production (most common mode of reproduction)
- Fungal fragmentation: a portion of the mycelium splits from the body of the fungus, with the resulting fragment being capable of replication
- : a cell undergoes nuclear division and splits into two daughter cells capable of replicating
- 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
Main methods of plasmogamy
- 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 
- 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 
- 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
Secondary metabolites: compounds not essential to cellular life
- Antibiotics: e.g., by Penicillium chrysogenum, cephalosporins by Acremonium chrysogenum
- Aflatoxins by molds (e.g., Aspergillus flavus)
- (see “”)
- Ergot alkaloids: ergotamine by ergot fungus (e.g., Claviceps purpurea)
|Overview of the most important dermatophytes|
|Overview of the most important yeasts|
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Pathogen: Aspergillus fumigatus
- Ubiquitous occurrence
- Some species produce aflatoxins