What is the functional biology of fungi relevant to mold allergy?

Updated: Dec 02, 2020
  • Author: Shih-Wen Huang, MD; Chief Editor: Harumi Jyonouchi, MD  more...
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Fungi have 2 basic structures. Yeast grows as single cells by means of central division of eccentric buds to form daughter units. Most other familiar fungi are composed of branching threads, 3-10 µm in width, termed hyphae. A mycelium is an aggregate of hyphae. Hyphae are modified to bear the simple reproductive parts of many microfungi and form the structural tissue of fleshy fungi (eg, mushrooms, puff balls).

In general, familiar allergenic molds reproduce asexually. However, 2 large and distinctive classes, Ascomycetes and Basidiomycetes, also produce innumerable sexual spores for atmospheric dispersion. In its life cycle, a single fungus organism produces both sexual and asexual spores from morphologically different structures respectively termed perfect and imperfect stages.

In considering known and potential allergens, 5 major classes of fungi have particular clinical significance: Oomycetes, Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.

Most molds require elemental oxygen during growth. Traces of formed carbohydrate are also essential. Vegetative hyphae of most fungi grow best at 18-32°C, and, although most become dormant at subfreezing temperatures, a few may sporulate below 0°C. At the other extreme, although 71°C is generally lethal for molds, certain types thrive at slightly cooler temperatures. Aspergillus fumigatus and Aspergillus niger tolerate a wide range of temperatures (see the image below).

Aspergillus. Aspergillus.

Atmospheric moisture affects not only the growth and fruiting of fungi but also the dispersion of spores and resultant prevalence. Spore counts typically rise with rainfall and fog and with damp, nocturnal conditions. Rain and dew splash also foster dispersion of slime spores. As a result, atmospheric recoveries of Fusarium, Phoma, Cephalosporium, and Trichoderma species peak with rainfall.

The reproductive units of many fungi are detached by direct wind scouring or wind-induced substrate motion. Such dry spore dispersal increases as airspeed rises and relative humidity falls, peaking often during summer afternoons. At such time, typical spores of Cladosporium, Alternaria, Epicoccum, Helminthosporium, Rhizopus, Aspergillus, and Penicillium species may also peak (see the images below).

Bipolaris. Bipolaris.
Cladosporium (Hormodendrum). Cladosporium (Hormodendrum).
Curvularia. Curvularia.
Dreschlera (Helminthosporium). Dreschlera (Helminthosporium).
Epicoccum. Epicoccum.
Penicillium. Penicillium.
Penicillium. Penicillium.
Stachybotrys. Stachybotrys.
Rhizopus. Rhizopus.

The circadian trends in changes of temperature, humidity, airspeed, and light intensity frequently interact to promote diurnal airborne spore levels. All data emphasize that regional vegetation strongly affects the local airborne spore levels.

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