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Mold of the month: Memnoniella echinata

Mold of the month: Memnoniella echinata (Rivolta) Galloway 1933

Picture left:
Memnoniella echinata from pure culture. Incubated for seven days at 25 °C on MEA agar. The round colonies are characterized by an outer white rim of sterile mycelium surrounding a dark center in which the black spores are formed. Centrally in the colony center rises sterile aerial mycelium (light colored). The dark coloration is caused by the rough and pigmented spore carriers together with conidiospores.

Picture right:
Memnoniella echinata from pure culture. Incubated for seven days at 25 °C on DG18 agar. Colonies grow considerably slower and overall pathological. No spores are formed within the usual incubation period of 7 - 10 days (specified in DIN ISO 16000:17 06-2010). The sterile mycelium of the colony is whitish to light brown. The example of Memnoniella echinata shows once again why a full medium (e.g. MEA) and a deficiency medium (DG18) are always used for mold investigations indoors (regardless of whether indoor air or material samples). The smaller colonies also DG18 allow a more precise quantification and the better grown colonies on MEA allow the identification of the present molds.

The mold species now known under the official name Memnoniella echinata was, according to available literature, first described in 1873. At that time still under the basionym (ancient Greek: original name) Penicillium echinatum. Common with the genus Penicillium is the phialid production of round spores in long chains, on phialides without underlying vesicles. Morphological and physiological studies finally resulted in a systematic assignment of the species to the genus Memnoniella. During the twentieth century, various attempts at systematic regrouping of this species followed, including into the genus Stachybotrys. Recent molecular data suggest a close relationship between the genera Stachybotrys and Memnoniella but they are clearly separate genera.

Memnoniella echinata is a mold with a worldwide distribution that has often been isolated from clothing or carpets indoors. The species is not one of the officially recognized moisture indicators in the interior (see mold guide from the Federal Environment Agency), but both the poor growth on culture media with osmotic stress (eg DG18) and the isolation of predominantly moist materials, suggest a high water demand of the species Memnoniella echinata. Thus, elevated detection of Memnoniella echinata may be indicative of increased indoor humidity. Concrete data on the aW value are not known.

According to TRBA 460, the mold species Memnoniella echinata 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 Memnoniella echinata and thus no known infections in humans are described in the current Altas of clinical fungi. Nevertheless, due to the production of various mycotoxins, prolonged exposure to Memnoniella echinata is not recommended (see Food and Indoor Fungi Sec. Ed.).

Particularly when processing particle traces (total spore measurements; DIN ISO 16000:20 11-2015), there is a risk that the spores of Memnoniella echinata may be confused with those of the species complex Aspergillus niger. The spores are similar in size and shape, however, experienced laboratories are able to determine the differences based on coloration and spore ornamentation. In this case the spores of Memnoniella echinata are counted to the spore type Stachybotrys and a written notice for clients is issued.

Light microscope image of spore carriers

Light microscope image of spore carriers

Light microscope image of spore carriers including mycelium of the mold species Memnoniella echinata (400x magnification). Starting from 7-day-old colonies, an adhesive film preparation was taken and stained with cotton blue. The blue staining of the mycelium is thus a preparation artifact which is desired to better identify the mycelium. It is clearly visible that the spore carriers (central in the image) are stained darker. Due to the magnification, the rough surface of the spore carriers cannot be seen. At the end of the spore carrier (conidiophores) there are usually 4 - 8 phialides on which the dark round spores are formed in chains.

Scanning electron micrograph sputtered with gold at 5159x magnification

Scanning electron micrograph sputtered with gold at 5159x magnification

Scanning electron micrograph of Memnoniella echinata sputtered with gold at 5159x magnification. The rough stalk of the spore carrier starts in the center right of the image and extends across to the phialides (3 recognizable and a total of 5 on the spore carrier, starting from the spore chains). On the lower left side the long spore chains are recognizable. The spores are also rough. It is noticeable that the spores are dented on one side and sit on top of each other with the indentation. Mature spores are later completely round.

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