Mold of the month - Stachybotrys chartarum ((Ehrenberg) S. Hughes 1958)
The mold species Stachybotrys chartarum was already described in 1818 (at that time still under the name Stilbospora chartarum). In 1958, it was assigned to the genus Stachybotrys, which is still valid today and is supported by both morphological and molecular data (source: Mycobank.org; as of 29.07.2022).
Picture left: Stachybotrys chartarum from pure culture. Incubated for seven days at 25 °C on malt extract agar. The circular colonies often form aerial mycelium, which appears slightly "electrified". This effect is caused by a bundling of individual mycelial strands. These bundles are called synnemata across species. In the case of Stachybotrys chartarum, the synnemata fringe out at the upper end in numerous side branches, creating a feather-like structure. The conidiospores are covered by the aerial mycelium and can be seen as a dark brown to black base in the center of the colonies.
Picture right: Stachybotrys chartarum from pure culture. Incubated for seven days at 25 °C on DG18 agar. A strongly inhibited growth of Stachybotrys chartarum is recognizable on DG18 agar, as the colonies are considerably smaller for the same incubation time (in direct comparison to 1a). In addition, the colonies often remain sterile, making morphological identification impossible. Thus, Stachybotrys chartarum represents an example of why, according to DIN ISO 16000-18:2012-01, two different culture media must always be sampled at one sampling point when examining culturable molds.
For a long time, the species Stachybotrys chartarum played a minor role in the analysis of indoor-relevant molds. Since the species grows relatively slowly, it is often strongly underrepresented in cultivating indoor air measurements because it is displaced by fast-growing genera (e.g. Aspergillus and Penicillium). Only with the introduction of the analysis of the total number of spores in the room air (particle traces according to DIN ISO 16000:20 or WTA Instruction Sheet 4-12) did the easily recognizable, dark spiked spores typical of moisture damage gain immense attention.
With an aw-value of 0.94, the mold species Stachybotrys chartarum is now one of the recognized moisture indicators in indoor areas, which generally only occur when the material is heavily soaked (mold guide from the Federal Environment Agency). An infestation by Stachybotrys chartarum is frequently to be found at dark and draught-poor places (behind big cabinets or on the back of plasterboard-walls) (Food and Indoor Fungi Sec. Ed.).
Stachybotrys chartarum is not only known for its high-water requirements but was also long regarded as a "dangerous" mold in indoor areas due to its sometimes strong mycotoxin production. According to recent findings, however, Stachybotrys chartarum is classified in biological risk group 1, which is why no increased risk of infection for humans is to be assumed (TRBA 460). An entry in the Atlas of Clinical Fungi (4th edition) is also missing. Nevertheless, mycotoxins on contaminated materials should not be underestimated and exposure should be avoided if possible. What is interesting about the mycotoxins is that there are four main groups produced, divided evenly between two chemotypes. Stachybotrys chartarum chemotype S produces satratoxins and roridins, while chemotype A produces atranones and trichodermins (Food and Indoor Fungi Sec. Ed.).
At 100x magnification under the light microscope
Overview of mycelial plexus including spore carriers and spore clusters of Stachybotrys chartarum at 100x magnification. Spores of Stachybotrys chartarum are dark brown to black, ellipsoid, and strongly spined. The spores are formed in clusters on club-shaped phialides, which are typically in groups of four to ten at the end of an unbranched spore carrier. The spore clusters may be covered by a small droplet. This glues the individual spores and the spore carriers together, which influences the "anemochory" (aerial dispersal).
At 7120X magnification under SEM
Spore carriers and spores of Stachybotrys chartarum sputtered with gold under vacuum at 7120x magnification under a scanning electron microscope (SEM). The spore carriers have a very rough surface, especially at the tip to the phialides (warty pocks in the SEM image). Both the ellipsoid spores (upper left in the image) and the spore carriers (lower right in the image) are sunken and dented by the preparation. This is due to the preparation for the SEM images.
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