Chaetomium globosum Kunze 1817
Why so curly?
The genus Chaetomium provides a vivid example to explain two central concepts of microbiology and at the same time to illustrate one of the most impressive differences compared to other molds. As can be seen in the pictures (3-4), the spores of the genus Chaetomium develop in so-called perithecia - these are spherical or bottle-shaped fruiting bodies (ascoma) in which the spores (asci) are formed. These are closed fruiting bodies and not spore carriers as found in the genera Penicillium, Aspergillus or Scopulariopsis (see definition Food and Indoor Fungi Sec. Ed. 2019).
Picture 1) Front view of a pure culture of Chaetomium globosum on MEA agar, incubated at 25 °C for ten days. The typical olive-green coloration of the perithecia in the center of the colony is characteristic for morphological identification.
Picture 2) Front view of a pure culture of Chaetomium globosum on DG18 agar, incubated at 25 °C for ten days. The colony appears whitish to slightly brown overall and is significantly smaller than on MEA. This is due to the fact that no perithecia are formed on DG18 agar, which provide the coloration in the MEA image ( Picture 1).
In contrast to spore carriers, which release spores directly into the environment, perithecia are closed fruiting bodies in which the ascospores are initially formed and mature. This closed structure prevents the spores from being easily dispersed by the wind (anemochory), which is often the case with other genera such as Penicillium. The ascospores are protected from abiotic and biotic influences during their maturation within the perithecium.
This is where the so-called perithecia hairs come into play. These rigid appendages, which grow on the outside of the fruiting body, resemble wavy or curly hairs in shape. It is conceivable that the perithecium hairs serve to catch insects so that their movements cause the perithecium to burst open (see Picture. 5; a mechanically pushed open perithecium). When the perithecium bursts open, the insect can simultaneously act as a vector for the spores (zoochory). This hypothesis could be supported by the fact that Chaetomium globosum forms comparatively large spores, which cannot be transported through the air as easily as the spores of Aspergillus or Penicillium (a partial reason why Chaetomium is underrepresented in some air measurements).
Otherwise, the species Chaetomium globosum in particular is a nice example of applied mycology. Various research groups from China are currently looking into the possibility of using Chaetomium globosum as a natural pesticide. Studies have shown that plants in which Chaetomium globosum is endophytic (i.e. living inside the leaves) are less susceptible to other fungal infections. In addition, Chaetomium globosum appears to be a local endophyte that occurs exclusively in the leaves and does not spread via the seeds of the host plant (in contrast to a systemic endophyte). This characteristic facilitates the control of this “biocontrol agent” as it only occurs on the treated plants (Microbiological Research Volume Ed. 2010 )
The advantage of endophytes is that fungi that occur endophytically in living leaves can break them down after the leaf has died. This gives them direct access to valuable nutrients, which increases their chances of survival and growth.
Medical relevance:
According to TRBA 460 (2016:07), the mold species Chaetomium globosum is classified in biological risk group 1. However, TRBA 460 refers to a different risk for immunocompromised people, as well as a possible allergenic effect and possible plant pathogenicity. According to the Atlas of Clinical Fungi (4th edition, 2020), Chaetomium globosum can cause infections of the lungs and lymph nodes in immunocompromised patients.
From the lowest magnification to the highest magnification
Picture 3) Close-up of matured perithecia of a Chaetomium globosum pure culture on MEA agar after more than 14 days of incubation. The perithecia are only fully mature after this time.
Picture 4) Light microscope image of Chaetomium globosum at 100x magnification (additionally digitally enlarged). The oval perithecia are easily recognizable by their brown colouring and the long, slightly wavy perithecial hairs. The bottle-like shape of the lower left fruiting body, with the flattened and pointed side, is particularly clear.
Picture 5) Light microscope image of Chaetomium globosum at 400x magnification (additionally digitally enlarged) of a mechanically opened perithecium. The opened perithecium with its perithecial hairs is clearly visible on the image. The brownish, lemon-shaped ascospores are clearly visible in the center of the image.
Picture 6) Scanning electron micrograph of Chaetomium globosum, sputtered with gold, at about 742x magnification. The perithecium was either mechanically crushed during the preparation process or damaged during the vacuum sputtering process. The rough surface of the perithecial hairs is clearly visible.
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