Classification of Fungi

The fungi are classified on the basis of their mode of sexual reproduction; however, the sexual stages can be difficult to induce and are rarely observed in clinical specimens. Most fungi reproduce by forming conidia through mitosis, during which the chromosome number remains the same. The descriptions of species deal principally with various asexual structures, and the fungi are identified on the basis of the morphologic characteristics of the hyphae, yeasts and conidia which may form on specialized conidiophores, on the sides or ends of non specialized hyphae, or from a hyphal cell. Specialized names have been given to each developmental form of conidia. Some pathogenic and many nonpathogenic fungi respond to environmental stimuli and can exhibit different morphologic forms. Demonstration of the two phases can be key diagnostic criteria. Reactivity in biochemical tests is used to identify yeasts and some other fungi, because the sexual stages of fungi are so seldom seen and because identification is based on asexual stages, the organisms will often have two formal names (e.g. Filobasidiella neoformans and Cryptococcus neoformans for the sexual and asexual stages. respectively). The names of the asexual stages are used commonly.
The fungi are classified as follows:

Ascomycotina

(Also know as the ascomyces)
Sexual fusion results in a sac, or ascus, containing the meiotic products as four or eight spores (ascospores). Asexual spores (conidia) are borne externally at the tips of hyphae.
Examples are trichophyton (Arthroderma), Microsporum (Nannizzia), Blastomyces (Ajellomyces).

Basidiomycotina (the basidiomyces)

Sexual fusion results in formation of a club shaped organ called a basidium, on the surface of which are borne the four meiotic products (basidiospores).

Deuteromycotina (the imperfect fungi)

This is not a true phylogenetic group but rather an artificial class into which are temporarily placed all forms in which the sexual process has not yet been observed. Most of the members of the group resemble ascomyces morphologically. Epidermophyton, Sporothrix, Candida species are the examples.

Zygomycotina (the phycomyces)

Mycelium usually nonseptate; asexual spores produced in indefinite numbers within a structure called a sporangium. Sexual fusion results in formation of a resting, thick-walled cell termed a zygospore. The examples are rhizopus nigrams (opportunistic pathogens only).

 

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