Mold of the month: Syncephalastrum racemosum (F. Cohn ex Schroeter 1886)
The mold species Syncephalastrum racemosum, due to its distribution in tropical or subtropical regions, is not necessarily one of the typical indoor molds in Germany (Compendium of soil fungi Sec. Ed. 2007). However, this species is frequently found on spoiled food and is also introduced into temperate climates in the course of global food transport chains. The ability to maintain vital spores under anaerobic conditions also allows dissemination via vacuumed and dried foods.
Picture left: Syncephalastrum racemosum from pure culture. Incubated for seven days at 25 °C on MEA agar. The circular colony is distinguished by a light outer rim surrounding a dark center. The dark gray area results with increasing age of the colony and the associated production of spore carriers. Due to the comparatively high growth rate, the maximum colony diameter of 9 cm is already reached within the 7-day incubation. Colonies of this size lead to a reduced evaluability of mold samples, since other molds are masked and thus morphologically not identifiable.
Picture right: Syncephalastrum racemosum from pure culture. Incubated for seven days at 25 °C on DG18 agar. Three adjacent colonies can be seen in the image. Each with darker center and light sterile border. In direct comparison to the colony growth on MEA agar, the smaller colony size is noticeable. The glycerol contained in the DG18 agar causes inhibited growth in this species, decreasing its ability to compete with other species.
In the natural environment, Syncephalastrum racemosum is found in soil, litter, or manure samples and thus is considered a destructive species. However, under laboratory conditions, it has also been found to have mycoparasitic properties (Compendium of soil fungi Sec. Ed. 2007). Unlike comparable zygomycetes, Syncephalastrum racemosum can grow even with comparatively low water availability. With a described aW value of 0.84, this species is one of the xerophilic molds that may well indicate increased humidity (Food and Indoor Fungi Sec. Ed. 2019).
Despite its ability to grow at temperatures around 40°C, Syncephalastrum racemosum is not known to have human pathogenic properties (Atlas of clinical fungi 4th ed.). According to TRBA 460, this species is classified in biological risk group 1 (risk of human disease is considered unlikely).
Light microscope image of two sporangia carriers
Light microscope image of two sporangia carriers of Syncephalastrum racemosum. Several smaller sporangia can branch off laterally from one main sporangium. The mycelium and the spore carriers are rarely if ever septate. The central vesicles are brown to dark gray in color in the colony. The bluish appearance in the image is an artifact due to preparation with cotton blue. On the vesicles stand circularly the so-called merosporangia in which merospores are formed. In young merosporangia, the merospores are surrounded by a common sheath, which later dissolves and then allows release of individual spores. Merospores are round to oval depending on age.
At 1193X magnification under SEM
Sporangia carrier of Syncephalastrum racemosum at 1193x magnification under scanning electron microscope (SEM) sputtered with gold under vacuum. In some cases, the individual merospores can be clearly seen, in which the shell has already dissolved. Younger sporangia carriers do not yet have mesospores, which is why the envelope of the mesosporangia is dented by the vacuum. The surface of the spore carriers is smooth. All structures are slightly dented. This is artifact of vacuum preparation and sputtering.