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Mold of the month: Trichoderma viride

Mold of the month: Trichoderma viride (Persoon 1794)

The mold species Trichoderma viride was first described in 1794 in the "Neues Magazin der Botanik". By the standards of the time, it was customary to publish innovations in the field of mycology in botanical journals. Current publications appear, thematically clean, in specific journals of mycology or botany.

Picture 1a) Trichoderma viride from pure culture. Incubated for seven days at 25 °C on MEA agar. A single greenish-yellow colony fills the entire agar plate. The center is generally darker green in color and becomes yellowish toward the outer edges. The thinly formed aerial mycelium can be seen as a slightly grayish down. The concentric growth of the colony can be recognized by the different colorations.

Picture 1b) Trichoderma viride from pure culture. Incubated for seven days at 25 °C on DG18 agar. A single whitish colony is growing in the center of the plate. The colony grows considerably smaller than on MEA agar and remains sterile at least within the seven day incubation. That is, no conidiospores are formed, which account for the colony's coloration. The colony remains flat and does not form aerial mycelium.

A characteristic that occurs in the entire genus Trichoderma and has undoubtedly already played a decisive role in the description is the very high growth rate. Trichoderma viride and also other members of the genus, under suitable conditions (25 °C on MEA), are quite capable of reaching a colony diameter of 6 - 9 cm after only four days of incubation. This means that the standard agar plates are completely overgrown after half of the incubation time of seven days prescribed by DIN ISO 16000-17:2008. This makes the evaluation of MEA agar plates immensely difficult, as other molds are overgrown and the necessary morphological criteria cannot be recorded. In addition, some Trichoderma species are able to suppress and partially decompose other molds to use them as an additional nutrient source. These two properties ensure that detection of Trichoderma on MEA agar plates can lead to underdetection of other molds.

The osmotic stress of DG18 agar plates sensitively slows down the genus Trichoderma, allowing for better evaluation (exemplified by the growth of Trichoderma viride). Due to the sterile growth of Trichoderma on DG18 agar plates, the identification of these molds is only possible on MEA plates. This illustrates why both DG18 and MEA measurements are important in the analysis of indoor molds.

In the routine analysis of molds in interiors, the genus Trichoderma plays a role in the evaluation for the reasons mentioned, but because these are typical molds on rotting wood or fruit, their detection plays a rather subordinate role in the evaluation of interior damage. For this reason, and taking into account that the more than 100 species described today can often only be reliably distinguished at the molecular level, the representatives of this genus are subsumed under Trichoderma spp. in routine analysis.

When examining particle traces (total spore measurements) or adhesive film preparations, the spores of the genus Trichoderma can sometimes be assigned to the spore type Aspergillus / Penicillium. A doubtless assignment is not always possible without spore carriers. The slightly elongated conidiospores may correspond to the spore type Aspergillus / Penicillium, depending on their orientation in space. The spore type subsumes a number of mold genera whose spores can be described as small and round. The characteristics in the direct microscopic methods are often insufficient to determine the genus beyond doubt.

According to TRBA 460, the mold species Trichoderma viride is classified in biological risk group 1. Thus, there is no known increased risk of infection for humans. In line with this, no entries for Trichoderma viride and thus no known infections in humans are described in the current Altas of clinical fungi. Concrete data on the aW value and thus the probable water demand, are not known.

Light microscopic image at 1000x magnification

Light microscopic image at 1000x magnification

Light microscopic image of Trichoderma viride at 1000x magnification. The image was digitally enlarged afterwards to make the structures to be described recognizable. Along a spore carrier (protruding from below into the center of the image) 2 - 4 whorls of phialides are formed in regular intervals, which are separated from the spore carrier by a basal septum. The phialides are bulbous and slightly tapered towards the top. At the end of the phialides the roundish conidiospores are formed. These are visible here in the picture as dark blue circles with blackish margins. The conidiospores have a rough ornamentation (surface structure).

Scanning electron micrograph sputtered with gold at 9674x magnification

Scanning electron micrograph sputtered with gold at 9674x magnification

Scanning electron micrograph of Trichoderma viride sputtered with gold at 9674x magnification. The round spores are depressed by the process of sputtering and by vacuum (preparation artifact). Normally, the spores are round, without a distinct dent. The surface structure of the conidiospores can be seen. The spores are in small clusters on the whorled positioned phialides (here hidden by the spores).

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