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Mold of the month: Aspergillus clavatus

Mold of the month - Aspergillus clavatus (Desmazières J.B.H.J 1834)

This globally occurring mold species is usally found on rotting organic waste, as well as in soil and on manure. The species plays a comparatively minor role indoors. Due to its low water activity, Aspergillus clavatus can also occur on dry substrates and is therefore classified as a xerophilic mold (Food and Indoor Fungi sec. ed.). Interestingly, Aspergillus clavatus is alkali-tolerant and thus can grow on alkaline substrates. Thus, the alkaline building materials commonly used indoors have only a limited preventive effect on Aspergillus clavatus. Presumably, Aspergillus clavatus influences the pH of its environment by releasing acidic substances (growing on CREA culture media; Food and Indoor Fungi sec.ed.).

Picture left: Aspergillus clavatus from pure culture. Incubated for seven days at 25 °C on MEA agar. The circular colony shows an outer white mycelial ring (concentric growth zone around the colony). In the center, the eponymous cobs are formed, on which the conidiospores are produced. As they mature, the cobs become longer and more intensely blue in color. The individual cobs stand comparatively loosely next to each other and not densely packed.

Picture right: Aspergillus clavatus from pure culture. Incubated for seven days at 25 °C on DG18 agar. Close-up of the characteristic cobs. Some cobs visible in the center of the image are shown laterally and, at 50x magnification, clarify the shape and color relevant for identification. Younger cobs can sometimes still be roundish (marginal area of the colony). Contrary to the molds presented so far, the morphological differences between the MEA and DG18 agar are rather small (growth rate is slightly different, for example).

According to the current TRBA 460, Aspergillus clavatus is to be classified in biological risk group 1. Nevertheless, increased exposure can lead to allergic reactions and severe inflammation in the lungs. Since these often occurred in malt processing, the condition is also known as malt worker's lung (exogenous allergic alveolitis). Neurological damage has been noted in animals following infection (Atlas of clinical fungi 4th ed.).

The images of Aspergillus clavatus shown indicate that even with very careful preparation, the spore chains detach from the spore carrier and readily disperse into the room air. These spore chains, in turn, rapidly disintegrate into individual spores. Therefore, the detection of spore chains in direct microscopic methods (e.g.: particle traces and adhesive film preparations), can be an indication of a spore source in the environment of the sampling site.

The spore carriers of the genus Aspergillus generally have round vesicles at the tip, the shape of which is visually reminiscent of heads. Therefore, the genus Aspergillus runs colloquially also under the designation "head fungi". In the species Aspergillus clavatus, this actually round head is deformed into an elongated cob. This characteristic form serves not only for the fast and safe morphological identification, but is at the same time also name-giving for the species (lat.: clavatus = club-shaped, colbous). The cobs only develop their characteristic shape as they mature. Morphological and molecular data indicate that there are two variations within the species. Aspergillus clavatus var. clavatus and var. major. The latter variation is slightly larger (; as of 08/22/2022).

At 100x magnification under the light microscope

At 100x magnification under the light microscope

Light microscope image of a flask of Aspergillus clavatus at 100x magnification under oil. The intense blue coloration is due to exposure to the dye cotton blue. The spores are formed on uniseriate phialides, which extend around the cob. The spores are usually in long chains on the phialides, and these are lost during preparation. In order to microscope the flasks as undamaged as possible, they were plucked from the colony with Dumont forceps and microscoped individually.

At 1294X magnification under SEM

At 1294X magnification under SEM

Spore carrier and individual spores of Aspergillus clavatus sputtered with gold under vacuum at 1294x magnification under scanning electron microscope (SEM). The flask has small warts at its base (towards the mycelium) over a short distance. The spore carrier itself is subsequently without structure and rather smooth. The recognizable phialides and spores are slightly dented. This is an artifact of vacuum preparation and sputtering. Spores and phialides are densely arranged around the flask.

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