Microascus paisii (Pollacci) Sand.-Den (2015)
If you search the internet for interesting entries on the mold species Microascus paisii, your initial curiosity will quickly be followed by considerable disappointment. Although Microascus paisii plays a relevant role in the assessment of indoor damage, this species seems to play a rather minor role in other scientific fields. The most frequent mentions in the literature deal with the revision of taxonomy based on molecular data and the discovery of new species within the genus Microascus. A few publications deal with the effect of fungicides, but on the whole, the species and, associated with it, the genus Microascus play a minor role in science. (As of August 2025)

One of many
But Microascus paisii is not alone in this. Because, to be honest, given the sheer number of described and undescribed species of fungi, it will be difficult to ever fully investigate them all and subject each fungus to a thorough examination of its usefulness to humanity or the environment. Currently, around 150,000 fungi are taxonomically described and accepted. Based on various models, the total diversity is estimated at 2 to 4 million species in conservative circles (Hawksworth and Lücking 2017). Others assume a total diversity of more than 10 million species. We will probably never know exactly how many species there are. Not only does the difficulty of detection due to their small size or cryptic species (morphologically indistinguishable) play a decisive role here, but also the fact that well-known species concepts for microorganisms such as bacteria, yeasts, and molds do not work as cleanly as science would like. Molecularly distinguishable species are often not as strongly separated from each other by various reproductive barriers as we might be accustomed to seeing in mammals and some plants. This leads to hybridization occurring much more quickly. The associated recombination of traits leads to an enormous evolutionary advantage in the medium term, as microorganisms are able to adapt much more quickly to changing environmental conditions. However, in the long term, it also leads to the emergence of ever new fungal species.
When assessing indoor damage in terms of microbial contamination and interpreting the species and genus spectrum determined by laboratories, it must always be taken into account that this is morphological data. Due to methodological limitations, this data is subject to an incalculable degree of uncertainty, meaning that some species are subsumed. Although molecular data refine the taxonomic resolution, it is not yet established in Germany to such an extent in the assessment of indoor damage that its results are widely accepted. In addition, essential metadata such as water requirements, temperature spectrum, or nutrient preferences are missing for many fungi, so that a name alone is not very useful.
Picture 1) Front of a Microascus paisii pure culture on DG18 agar incubated for ten days at 25 °C. Morphological identification is possible using direct microscopy. The colony is significantly smaller than on MEA and colorless. Fewer typical spores are formed on the annelids. Mycelium and spore carriers are found more in the center of the colony, whereas the edge often grows yeast-like.
Picture 2) Front side of a Microascus paisii pure culture that was incubated on MEA agar for ten days. The colony is significantly larger and shows no yeast-like cells at the outer edge. The white and airy mycelium forms numerous spore carriers. Even older colonies turn grayish due to increasing spore production.
Picture 3) Front view of a pure culture of Microascus paisii incubated on OA agar for ten days. The colony forms yeast-like cells at the edge, whereas in the center, distinct and, above all, gray-colored aerial mycelium stands out. The gray coloration increases with the age of the colony and spore maturity.
Taxonomy:
The mold species Microascus paisii received its currently valid and recognized name (source: Mycobank.org; as of July 2025) recently in 2015, when Sandoval-Denis and colleagues published a revision of the genera Scopulariopsis, Microascus, and other closely related molds based on molecular genetic data. Using a comprehensive multigene analysis (4 markers), more than 100 species and strains were examined, and old genera were confirmed or new ones established based on monophyletic and polyphyletic divisions. The species was first described by Pollacci in 1921, only to be described and published again in 1928 and 1934 by the same author, but with a completely different genus assignment. This recurring reclassification to other genera shows how enormously difficult it is to distinguish between the genera Scopulariopsis, Microascus and other closely related genera on a purely morphological basis. Interesting fact, the first descriptions of Microascus paisii were published primarily in medical journals, which is probably due to the pathological relevance of this species.
Routine analysis:
In routine analysis, the species Microascus paisii is not officially recognized as an indoor moisture indicator (mold guide from the US Environmental Protection Agency 2024; Food and Indoor Fungi 3rd Edition). However, the laboratory's personal experience, based on the frequently observed association with recognised moisture indicators such as the genus Acremonium spp. (partly newly designated as Sarocladium) or the species complex Aspergillus versicolor, suggests an aW value of approximately 0.8. Due to its high water requirements and its medical prevalence, this species is one of the molds whose detection indoors is highly relevant for the assessment of building pollutants.
However, this method of identification can be tricky, as Microascus paisii is very easy to identify when fully grown, but usually requires an incubation period of 10 to 14 days to reach this stage. In routine analysis, samples are rarely incubated for such a long time. This is simply because other fast-growing molds fill the plate so much during this time that little of the comparatively small Microascus paisii colonies can be seen. This poses a challenge for laboratories, which need to be able to recognize and correctly identify the colonies and spores at a very early stage of development. To make matters more difficult, Microascus paisii grows poorly on the routine medium DG18, so that reliable detection is often only possible using MEA agar (Figures 1, 2, and 3). This is yet another example of why it is essential to sample both culture media (MEA and DG18) in routine mycological analysis. However, it is essential to identify this mold species with certainty, as closely related and morphologically similar species (e.g., from the genus Scopulariopsis) differ significantly in their physiological and pathogenic properties, which could lead to misjudgments of the situation on site. It should be noted that Microascus paisii cannot currently be distinguished morphologically from the closely related species Microascus atrogriseus. Molecular data is required for this.
Another reason for the unequivocal and reliable detection of Microascus paisii is the regularity of its detection in indoor samples. Regardless of whether adhesive film samples are taken from material surfaces, slit and hole impaction of indoor air or building materials, Microascus paisii is regularly detected. Particularly in non-cultivation methods such as slit impaction (total spore measurement/particle collection) and adhesive film samples, the detection of spore carriers and the orientation of the spores in the preparation is crucial, and even then, under optimal conditions, it is often only possible to achieve reliable identification at the genus level. Why does the orientation of the spores play such an important role? The spores (see Figure 4) have a flattened base and a slightly pointed bulge on the opposite side. If the spores are slightly tilted, they appear completely round and there is a risk of assigning the spores of the genus Microascus to the spore type Aspergillus / Penicillium. The criteria for this spore type are small, round spores that often still have small tips, but these are not always visible due to their spatial orientation.
Overall, the mold species Microascus paisii is found worldwide and has often been detected in soil or compost samples as well as indoors. This suggests that this species also plays a role as a saprophyte of dead organic material. Its detection in compost soils is supported by the thermophilic properties of this species, as its physiological growth optimum is reached at 30°C. Some strains can still grow relatively well at 37°C.
Medical relevance:
In order to determine the classification of Microascus paisii in biological risk groups according to TRBA 460 (2016:07), it is first necessary to query Mycobank for the valid taxonomic synonyms. This is because there is no entry in TRBA 460 under the currently recognized species name Microascus paisii. However, there is an entry under one of the valid synonyms, Scopulariopsis brumptii, which was the most widely used name for this species from 1935 to 2015. Please note that according to the current TRBA 460, Scopulariopsis brumptii is the synonym of today's Gliomastix murorum. This is probably an error, as Gliomastix murorum originated from the former Acremonium murorum. There is currently no detailed classification of Microascus paisii in any of the biological risk groups.
However, TRBA 460 is not the only source, and certainly not the most up-to-date source, for assessing the risk potential of mold fungi with regard to possible infection in humans. According to the Atlas of Clinical Fungi (4th Ed. 2020), Microascus paisii has been detected in a few cases of lower respiratory tract infection or in patients with compromised immune systems in whom the cellular response does not function correctly. Taking into account the syntax from TRBA 460, this allows a possible classification in risk group 1G to be derived (see, for example, the classification of the species complex Aspergillus versicolor).

Microscopic image at 1000x magnification
Various small spore clusters can be seen, consisting of spores without any noticeable external ornamentation. In the center of the image is a spore carrier with a total of five annelids, from which individual spores hang. Annelids are spore-producing cells that release these spores basipetally, through a common opening, in chains. Each spore that leaves the annelid leaves a ring at the end of the annelid. This effect causes annelids to become significantly larger with age, as several rings line up next to each other. Some may be familiar with the term annelid, as it was already introduced in the mold of the month for February Scopulariopsis brevicaulis. In contrast to the annelids of Scopulariopsis brevicaulis, these have a bulbous base. Some spores have a slightly pointed tip at the upper edge. The spores are flattened at the base. This is the point of attachment to the annelid or the subsequent spore.

Scanning electron microscope image with gold sputtering at approximately 8062x magnification
Scanning electron microscope image of Microascus paisii sputtered with gold at approximately 8062x magnification. The scale bar on the right side of the image indicates 10µm. A spore carrier with three annelids is shown in the center of the image. The spores just above the annelids clearly show their flattened base. No significant ornamentation of the spores is visible in the SEM. The dents and grooves on the spores are preparation artifacts.

Scanning electron microscope image with gold sputtering at approximately 5159x magnification
Scanning electron microscope image of Microascus paisii sputtered with gold at approximately 5159x magnification. The scale bar on the right side of the image indicates 10µm. A total of five spore carriers can be seen, with varying numbers of annelids attached to their ends. In addition, isolated spore chains can be seen. These clusters of spore carriers along a mycelium strand are typical of Microascus paisii. They are often formed in aerial mycelium strands.
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