Cytology and cell structure
Historically, these organisms were assigned to the kingdom Fungi (Mycota), but later some of them were reassigned to the kingdom of stramenopiles (Chromista). Today, microbiology studies True Fungi — the kingdom Eumycota. They are ubiquitous in nature, especially in the soil.
The internal structure of a fungal cell is typical for eukaryotes: a well-defined nucleus with a membrane, cytoplasm, and organelles are present. However, the outer layers have striking differences:
- Cell wall: rigid and multilayered. It is composed of polysaccharides (chitin, mannans, glucans, cellulose), as well as lipids and proteins. Some species are capable of forming a capsule.
- Cytoplasmic membrane (CPM): contains glycoproteins, phospholipids, and, critically for medicine, ergosterols. Ergosterol, which differs from human cellular cholesterol, serves as the main target for many pharmacological antifungal drugs.
Vegetative fungal cells behave as Gram-positive (G+) upon staining and are non-acid-fast.
Morphological forms: molds and yeasts
The fungal body is termed a thallus. Structurally, there are two main forms of growth: hyphal (mycelial) and yeast.
Hyphal fungi (molds) The basic structural unit of molds is a hypha — a thin, branching filament ranging from 2 to 100 µm in thickness. A collection of hyphae forms a mycelium. Hyphae are divided into:
- Vegetative: grow into the nutrient substrate and provide nourishment.
- Aerial (reproductive): rise above the surface and bear reproductive structures.
According to the structure of the mycelium, fungi are divided into:
- Lower: hyphae lack cross-walls. The entire thallus is a single giant multinucleated cell (coenocytic mycelium).
- Higher: hyphae are divided by special cross-walls with pores — septa.
Yeast and yeast-like fungi These are unicellular organisms, most often oval in shape. True yeasts reproduce both sexually and asexually (by budding or fission). Unicellular growth during budding can lead to the formation of pseudohyphae — chains of elongated cells that lack the common sheath inherent to true mycelium (this is called pseudomycelium). Yeast-like fungi are morphologically indistinguishable from true yeasts, but completely lack sexual reproduction (they reproduce only vegetatively). In medical practice, they are often grouped together as "yeasts".
Phenomenon of dimorphism
Some species exhibit dimorphism — a unique ability to switch their growth form between mycelial and yeast-like. This process is strictly dependent on ambient temperature:
- At room temperature (usually when cultured on nutrient media), the fungus grows in a hyphal form and forms a mycelium.
- Upon entering the human body (at a temperature of 37 °C), it transforms into the yeast phase, which is often associated with its pathogenic properties.
Reproduction and sporulation
Fungi are capable of reproducing in two main ways. Fungal stages that reproduce sexually (with the formation of gametes and sexual spores) are called teleomorphs. Stages utilizing asexual reproduction (fragmentation of hyphae, budding, asexual spores) are called anamorphs.
Asexual spores are classified by their site of formation:
- Endogenous (sporangiospores): mature inside a special enclosed structure — the sporangium. Typical for the genus Mucor.
- Exogenous (conidia): form openly at the tips of spore-bearing hyphae — conidiophores. Found in Penicillium and Aspergillus.
The mechanisms of conidia formation vary: arthroconidia are formed by even septation and fragmentation of the hypha, while blastoconidia result from budding. By size, small unicellular microconidia and large multicellular macroconidia are distinguished.
To survive adverse conditions, fungi form resting forms: thick-walled chlamydospores (chlamydoconidia) and sclerotia (hard cellular masses covered by a dense coat).
Taxonomy and medical significance
Globally, the kingdom Eumycota is classified according to the presence of a sexual process. This division underlies the distinction between perfect and imperfect fungi.
Perfect fungi possess a sexual mode of reproduction. Three types are medically significant:
- Zygomycetes (Zygomycota)
- Ascomycetes (Ascomycota)
- Basidiomycetes (Basidiomycota)
Imperfect fungi are capable of asexual reproduction only. They are isolated into a formal (conditional) group — Deuteromycetes (Deuteromycota).
Briefly: answers to topical questions
Which specific classes (phyla) of fungi belong to higher fungi, and which to lower fungi?
Lower fungi include zygomycetes, chytridiomycetes, and oomycetes, whereas specific classes of higher fungi are not listed in the sources.
- Zygomycetes (Zygomycota) — a phylum/class of lower fungi, which includes Mucor.
- Chytridiomycetes (Chytridiomycota) — aquatic lower fungi.
- Oomycetes (Oomycetes) — lower fungus-like organisms.
Higher fungi are characterized by the presence of septate hyphae, but their exact taxonomic composition is not specified in the text.
Which pharmacological groups of antifungal drugs target ergosterol?
Ergosterol or its biosynthesis is targeted by three main pharmacological groups of antifungal drugs.
- Polyenes (e.g., amphotericin B) — bind directly to ergosterol molecules in the cell membrane, disrupting its integrity.
- Azoles (including imidazoles) — inhibit the enzyme 14-demethylase, causing impaired ergosterol synthesis.
- Allylamines (e.g., terbinafine) — inhibit the enzyme squalene epoxidase, leading to ergosterol deficiency.
These groups block biosynthesis at various stages or act on the finished molecule.
What special staining methods are used in microbiology to visualize fungi in tissues?
For the visualization of fungal elements in tissues and histological specimens, the sources indicate the following special staining methods.
- PAS reaction (periodic acid-Schiff stain, Schiff's reagent stain) — used to visualize fungal elements that are poorly distinguishable with standard hematoxylin and eosin staining.
- Gomori methenamine silver stain / Grocott stain — used in histological preparations; for histoplasmosis, it is noted to provide the clearest picture and stains yeast cells black-brown; also used to detect cyst wall polysaccharides.
- Toluidine blue stain — indicated as a special method for detecting cyst cell walls.
To avoid confusing mycelial structure: "Lower" fungi evolutionarily did not reach the point of building "walls" (septal cross-walls), which is why they represent one huge common cell (coenocytic mycelium).
Class and exam questions
What is the main difference between the fungal cell wall and the bacterial cell wall?
The fungal cell wall consists predominantly of polysaccharides (chitin, glucans, mannans), whereas in bacteria the basis is peptidoglycan. In addition, fungi are eukaryotes.
How does pseudomycelium differ from true mycelium?
True mycelium consists of hyphae with a common cell wall. Pseudomycelium is simply a chain of elongated budding yeast cells that have not separated after division and lack a common sheath.
Why is ergosterol so important in medical microbiology?
Ergosterol is present exclusively in the fungal cytoplasmic membrane (mammals have cholesterol there). This makes it an ideal selective target for antifungal drugs.
What are perfect and imperfect fungi?
Perfect fungi (Zygomycetes, Ascomycetes, Basidiomycetes) have a sexual reproduction stage in their lifecycle. Imperfect fungi (Deuteromycetes) reproduce exclusively asexually.
What else to study on this topic
- Mechanisms of action of antifungals targeting cell membrane ergosterol
- Detailed morphology of the genera Mucor, Penicillium, and Aspergillus
- Clinical significance of dimorphism in systemic mycoses
- Processes of sclerotium and chlamydospore formation
- Pathogenic representatives of imperfect fungi (Deuteromycota)
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