Cytology and Cell Structure
Historically, these organisms belonged to the kingdom Fungi (Mycota), though some were later reclassified under Chromista. Today, microbiology studies True Fungi under the kingdom Eumycota. They are widely distributed in nature, especially in soil.
The internal structure of a fungal cell is typical of eukaryotes: a well-defined nucleus with a nuclear envelope, cytoplasm, and organelles. However, the outer layers have distinct features:
- Cell wall: Rigid and multilayered. Its composition includes polysaccharides (chitin, mannans, glucans, cellulose), lipids, and proteins. Some species are capable of forming a capsule.
- Cytoplasmic membrane: Contains glycoproteins, phospholipids, and, critically for medicine, ergosterol. Ergosterol, which differs from human cellular cholesterol, serves as the primary target for many pharmacological antifungal agents.
Fungal vegetative 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 growth forms: hyphal (mycelial) and yeast.
Hyphal Fungi (Molds) The basic structural unit of molds is the hypha, a fine branching filament 2 to 100 µm in width. A collection of hyphae forms the mycelium. Hyphae are divided into:
- Vegetative hyphae: Grow into the nutrient substrate and provide nourishment.
- Aerial (reproductive) hyphae: Rise above the surface and bear structures for reproduction.
According to mycelial structure, fungi are divided into:
- Lower fungi: Hyphae lack cross-walls. The entire thallus is a single giant multinucleated cell (coenocytic mycelium).
- Higher fungi: Hyphae are divided by specialized cross-walls containing pores, called septa.
Yeast and Yeast-like Fungi These are unicellular organisms, most commonly oval in shape. True yeasts reproduce both sexually and asexually (by budding or fission). Unicellular growth via budding can lead to the formation of pseudohyphae—chains of elongated cells that lack the common envelope characteristic of a true mycelium (termed pseudomycelium). Yeast-like fungi are morphologically indistinguishable from true yeasts, but completely lack a sexual mode of reproduction (they reproduce solely via vegetative means). In medical practice, they are often grouped together under "yeasts".
The Phenomenon of Dimorphism
Certain species exhibit dimorphism—the unique ability to switch their growth form between mycelial and yeast-like phases. This process is strictly dependent on ambient temperature:
- At room temperature (typically during culture on nutrient media), the fungus grows as hyphae and forms a mycelium.
- Upon entering the human body (at 37 °C), it transforms into the yeast phase, which is frequently linked to its pathogenic properties.
Reproduction and Spore Formation
Fungi are capable of reproducing through two main pathways. Fungal stages that reproduce sexually (involving gametes and sexual spores) are called teleomorphs. Stages utilizing asexual reproduction (hyphal fragmentation, budding, asexual spores) are called anamorphs.
Asexual spores are classified by their site of formation:
- Endogenous (sporangiospores): Mature inside a specialized enclosed structure, the sporangium. Typical of the genus Mucor.
- Exogenous (conidia): Form openly at the tips of spore-bearing hyphae called conidiophores. Found in Penicillium and Aspergillus.
Mechanisms of conidia formation vary: arthroconidia are produced by uniform septation and fragmentation of a hypha, whereas blastoconidia result from budding. Based on size, small unicellular microconidia and large multicellular macronidia (macroconidia) are distinguished.
To survive adverse conditions, fungi form resting forms: thick-walled chlamydospores (chlamydoconidia) and sclerotia (hard cellular masses covered by a dense rind).
Taxonomy and Medical Significance
Globally, the kingdom Eumycota is classified according to the presence or absence of a sexual cycle. This division underlies the distinction between perfect and imperfect fungi.
Perfect fungi possess a sexual mode of reproduction. Three types are clinically significant:
- Zygomycetes (Zygomycota)
- Ascomycetes (Ascomycota)
- Basidiomycetes (Basidiomycota)
Imperfect fungi are capable solely of asexual reproduction. They are grouped into a formal (artificial) category—Deuteromycetes (Deuteromycota).