Mold Pseudogymnoascus pannorum
Extreme properties for extreme research
To earn the title of model organism requires either a stroke of luck (see the article on Penicillium chrysogenum) or certain physiological peculiarities that are comparatively rare in nature. In the case of Pseudogymnoascus pannorum, the latter applies. This psychrotolerant mold is even capable of growing at temperatures below 0°C. This points to a few important and remarkable physiological characteristics.
Foremost are two things. On the one hand, it must be ensured that the water content in the cytoplasm does not freeze at such low temperatures and that the cells are not destroyed by ice crystals. On the other hand, psychrotolerant organisms require special enzymes that allow metabolic activity even at such low temperatures. As you may recall from biology class, metabolic activity/rate (catabolism and anabolism) is directly dependent on ambient temperature. Any uncoupling always requires the action of enzymes using energy. Since some viable strains have been found in Antarctic ice, on cave paintings, or in mine tunnels, the cold protection of Pseudogymnoascus pannorum seems to work quite well.
Because of the properties described, Pseudogymnoascus pannorum is considered one of the “extremotolerant” microorganisms and thus enjoys special standing in a variety of research projects. Several of the cold-active enzymes (special lipases and proteases) have already been identified, as well as melanin-like substances and trehalose, which, like detergents, prevent the cytoplasm from freezing.
In biotechnology, already-known enzymes and compounds are isolated and, for example, investigated as additives in detergents. There they enable energy-saving yet efficient use of 30°C wash cycles. The food industry is particularly interested in enzymes that allow the conversion of metabolic products during cold storage. From this, especially gentle food processing methods can be derived. On the other hand, psychrotolerant microorganisms also pose a particular hazard to hygiene in food cold chains, so understanding intracellular processes can lead to improved food protection. Last but not least, psychrotolerant microorganisms are relevant in the aforementioned bioremediation (see article on Cladosporium sphaerospermum). This involves the targeted breakdown of pollutants in the environment through the use of microorganisms. Since human economic activity also extends to cold regions of the planet, psychrotolerant microorganisms are needed in these regions for bioremediation.
A noteworthy downside of this mold is its frequent detection in archives and museums. Cold storage of artworks and records to maximize preservation appears to provide ideal conditions for Pseudogymnoascus pannorum and thus carries the risk of irreversible damage to valuable works. Routine, non-invasive surface sampling using adhesive tape preparations in accordance with VDI 4300 Part 13 (currently still in draft; as of 11.2025) is recommended to ensure early detection and timely measures.
Taxonomy:
As is often the case, physiological and morphological criteria played a decisive role in the naming of Pseudogymnoascus pannorum. Known under this name since 2013 (described by Minnis and Lindner in Fungal Biology 117), a molecular revision of the mold genus Geomyces also led to the renaming of this species. From 1976 to 2013 the species was known as Geomyces pannorum, with the genus Geomyces at that time apparently serving as a catch-all for psychrotolerant (cold-loving) molds. The species was first described in a scientific publication in 1824 as Sporotrichum pannorum.
The name components are derived from Greek. The syllable pseudes stands for “false,” gymnos for “naked,” and askos for “sac” or “bag.” The latter two parts, gymnos and askos, refer directly to morphological features. The Ascomycetes, to which Pseudogynmoascus pannorum belongs, are commonly called sac fungi because the ascospores (sexual spores after mitosis and meiosis) develop inside a long sac. The addition of “naked” may indicate that spore production in Pseudogymnoascus pannorum is relatively simple and that there are no elaborate sporophores or fruiting bodies.
The derivation of the species epithet pannorum (Lat. = rags or cloths) is less clear. There are different statements, some referring to the colony growing in a ragged or flappy manner and others suggesting that strains were frequently isolated from clothing. Neither of these claims can be verified based on in-house laboratory findings.
Routine analysis:
In routine analysis the species Pseudogymnoascus pannorum is not considered one of the clearly recognized moisture indicators in indoor environments (UBA mold guideline 2024), but with an aW value of up to 0.89 (Food and Indoor Fungi 3rd Ed.) it can nevertheless be assumed that a detection may be associated with elevated moisture. Because some strains can show slight growth even below 0°C (optimal 10–15°C), it is conceivable that increased detection occurs especially in moisture damage in cold rooms (e.g., basements). Under these conditions Pseudogymnoascus pannorum likely has a physiological advantage over other molds that prefer higher temperatures for growth. For laboratory analysis this context creates a crucial factor to consider when interpreting samples.
To avoid quantitative (number of colony-forming units) and qualitative (species and genus spectrum) undercounting, as many culturable molds as possible must be recorded during evaluation. At the normatively prescribed incubation temperature of 25 ± 3°C (DIN ISO 16000-17:2010-06), psychrotolerant molds are, however, systematically disadvantaged, which can lead to undercounts. This can be the case particularly on heavily populated culture media (> 100 CFU/plate; DIN ISO 16000-17:2010-06) and should always be taken into account by the commissioning parties when interpreting laboratory data. From the laboratory side this is difficult to counteract, because at the time when colonies are still quite small they can be counted well but cannot be identified with certainty. Therefore, an expert sampling strategy plays an important role in preventing sample overgrowth. The sampling volume should never be considered a fixed value but should be adjusted situationally. Laboratories often provide project-specific advice on this. If suspected, it can be advisable to perform a targeted incubation for psychrotolerant molds at 10–15 °C together with the partner laboratory (this may require additional impaction measurements).
Relevant for the assessment of indoor damage are, not least, the materials on which Pseudogymnoascus pannorum is detected. The relevant works (Mycobank.org (as of 11/2025) and Food and Indoor Fungi (3rd Ed. 2025)) list numerous and very diverse records. These can most likely be summarized as a preference for plant material, soil samples, fabric remnants and also paper fragments. Traditional building materials such as insulation or plasters have so far been described only very rarely. However, wallpaper can be a potential source due to paper fragments.
The described material sources point to a classic saprophyte on dead plant material and keratin. Interesting are reports of Pseudogymnoascus pannorum on antique glass. It is suspected that growth of this mold on glass (biofilm formation) leads to optical clouding, which is caused by secreted acids (e.g., oxalic acid and citric acid) (Food and Indoor Fungi 3rd Ed.). The strength of the glass is not affected.
Medical relevance:
According to current knowledge, Pseudogymnoascus pannorum is not associated with any relevant medical prevalence. Neither the current Atlas of Clinical Fungi (4th Edition 2020) nor TRBA 460 (2016:07) provide information on investigated and known effects of this mold species on humans. Only in Food and Indoor Fungi (3rd Ed.) is a detection of Pseudogymnoascus pannorum in superficial infections of skin and nails in humans mentioned. It is unclear, however, whether these reports possibly refer to Pseudogymnoascus destructans, a closely related species (see the agent of white-nose syndrome in bats).
Overview and front page on pure culture
Image 1) Overview of three Pseudogymnoascus pannorum pure cultures on DG18-agar incubated for seven days at 25 °C. The colonies are comparatively small and exhibit a slight yellow coloration. They show yeast-like growth. Only in the center of the colonies are slightly elevated mycelial strands aggregated into synnemata, on which occasional spores are produced.
Image 2) Front view of a Pseudogymnoascus pannorum pure culture incubated for seven days on MEA-agar. The colony is somewhat larger than on DG18 and also displays more pronounced mycelial and spore formation in the center. Overall, the colony grows in various shades of yellow and shows yeast-like growth at the margin.
Close-up at 10 days on MEA agar
Close-up of a Pseudogymnoascus pannorum culture after 10 days on MEA agar. The isolated synnemata appear slightly yellowish. Numerous small conidiophores branch off laterally from the synnemata.
4. Close-up image of the synnemata in the center of the colony after ten days on MEA agar
Close-up of the synnemata in the center of a colony of Pseudogymnoascus pannorum after ten days on MEA agar. Distinct thickenings are visible at the ends of the sporophores, indicating the typical shape of these spore-bearing structures as seen under the light microscope.
5. Light microscopic image at 400x magnification
Light microscopic image of Pseudogymnoascus pannorum at 400x magnification. In the case of Pseudogymnoascus pannorum, the "spore-bearing structures" are actually fertile hyphal strands that branch at their respective ends and fragment off viable spores at regular intervals. This is also called an arthroconidium. Short chains of 2–4 arthroconidia can form. The connecting hyphal fragments retract in old cultures or after the spores have been released.
6. Scanning electron micrograph sputter-coated with gold
Scanning electron microscope image of Pseudogymnoascus pannorum sputter-coated with gold. The scale bar at the right of the image indicates 5µm. The overall structure (arthroconidium) therefore measures approximately 25–30 µm. Visible is a single arthroconidium with numerous branching. The hyphal strands repeatedly show swellings from which the arthroconidia pinch off during colony maturation. The dimples on the spores and the flattened hyphal strands are preparation artifacts caused by vacuum and sputter-coating.
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