Taxonomy Features
The group Deuteromycota represents a formal (artificial) taxon. The main criterion for inclusion in this group is the absence of a sexual mode of reproduction. The term "formal" emphasizes that a sexual stage (teleomorph) potentially exists in these microorganisms, but has not yet been discovered by science.
In modern mycology, the term "mitosporic fungi" is increasingly proposed for use instead of the classic term "deuteromycetes". It more accurately reflects their mechanism of reproduction: the formation of asexual spores occurs exclusively through mitosis.
If a fungus from this group is suddenly found to have a sexual process during studies, its taxonomic status changes. The microorganism is excluded from deuteromycetes and transferred to another phylum — most often Ascomycota (ascomycetes) or Basidiomycota (basidiomycetes). At the same time, the fungus is assigned a new species name that corresponds to its discovered teleomorphic (sexual) form.
Morphology and Main Groups
A common morphological feature of most deuteromycetes is the presence of septate mycelium (divided by septa). They reproduce exclusively asexually, forming specialized structures known as conidia.
Several important categories are distinguished within this conventional group:
- Imperfect yeasts. These include the genera Candida, Cryptococcus, and Malassezia.
- Mould fungi (filamentous fungi). Represented by the genera Epidermophyton, Coccidioides, and Sporothrix, as well as the asexual stage of fungi of the genus Aspergillus.
- Dematiaceous fungi. These are dark-pigmented fungi, such as Phialophora and Fonsecaea.
Representatives of these groups cause numerous human diseases, including various mycoses, mycetoma, phaeohyphomycosis, and chromoblastomycosis.
Characteristics of Imperfect Yeasts (Using Candida as an Example)
The genus Candida is a classic representative of the group of imperfect yeasts (yeast-like fungi). They are the causative agents of candidiasis — an infectious lesion of the skin, mucous membranes, and internal organs.
Cells of fungi of the genus Candida are oval in shape, and their diameter varies within 2–5 µm. The main method of reproduction is division by budding.
Key structural features of the genus Candida:
- Formation of pseudomycelium (pseudohyphae). During budding, chains of elongated cells are formed. Unlike true mycelium, they lack a common cell wall.
- Ability to undergo dimorphism. Under certain conditions, they can form true septate hyphae.
- Formation of blastospores. The fungi form clusters of budding cells (blastospores).
An important aspect of differential diagnosis: true yeasts (belonging to ascomycetes) are capable of forming ascospores (sexual spores). Yeast-like fungi of the genus Candida never form ascospores. A distinct species feature of the causative agent Candida albicans is the ability to form chlamydospores — specific thick-walled resting spores resistant to adverse conditions.
Briefly: answers to topical questions
What types of asexual spores do various deuteromycetes form?
Various deuteromycetes form several types of asexual spores. The main structures of asexual reproduction include:
- Conidia — the main type of spores, including microconidia (e.g., pear-shaped) and macroconidia.
- Blastospores — clusters of budding cells characteristic of yeast-like fungi.
- Chlamydospores — thick-walled resting spores.
- Arthrospores — spores formed in old cultures and pathological material.
Fungi of this group reproduce exclusively asexually via mitosis.
What species of the genus Candida, besides C. albicans, are pathogenic to humans?
In addition to Candida albicans, clinically significant species include C. tropicalis, C. glabrata, C. krusei, C. parapsilosis, C. guilliermondii, and C. kefyr. The species C. glabrata was previously classified as Torulopsis glabrata. In complicated vulvovaginal candidiasis, C. glabrata and other C. non-albicans species are detected in 10–20% of female patients. Yeast-like fungi of the genus Candida cause lesions of the skin, mucous membranes, and internal organs.
On which nutrient media are fungi of the genus Candida cultivated?
Sabouraud medium, wort agar, liquid wort, and Czapek medium are used for cultivating the fungi. If fungal sepsis is suspected, liquid Sabouraud medium is used. C. albicans colonies are whitish-cream, convex, and round in shape. The fungi are cultivated for several days, as they grow slower than bacteria.
To remember the essence of the taxon, use the association: "Deuteromycetes (imperfect fungi) are the teenagers of the microworld: they are still imperfect (Fungi imperfecti), know nothing about sex (no sexual reproduction), and do nothing but divide (mitosis)."
Class and exam questions
Why are deuteromycetes called a "formal" taxon?
Because this is an artificially created group for fungi whose sexual stage has not yet been found. If it is found, the fungus leaves this taxon permanently.
What is the main difference between Candida yeast-like fungi and true yeasts?
True yeasts reproduce sexually and form ascospores. Candida belongs to imperfect fungi and does not form ascospores.
What is pseudomycelium?
These are chains of elongated cells formed during budding. The main difference from true mycelium is the absence of a common cell wall in these cells.
What synonym is increasingly replacing the term "deuteromycetes"?
The term "mitosporic fungi", as it indicates the formation of asexual spores exclusively through mitosis.
What else to study on this topic
- Pathogenesis and clinical forms of cutaneous and mucosal candidiasis.
- Clinical and diagnostic features of chromoblastomycosis and mycetoma.
- Dematiaceous fungi: pathogenicity factors of dark-pigmented forms.
- Conditions for chlamydospore formation in Candida albicans and their diagnostic significance.
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