Mold of the month: Talaromyces islandicus (Sopp) Samson et al 2011
Molds of the genus Talaromyces have long been included within the genus Penicillium. However, morphological characteristics such as the mustard yellow mycelium (often at the edge of colonies) and the metulae / phialides length ratio, in combination with molecular data form clear differential characteristics. The Talaromyces islandicus considered here was for a long time listed under the species name Penicillium islandicum (described 1912 by Sepp). The classification within the genus Penicillium is no longer legitimate today.
Picture 1a) Talaromyces islandicus from pure culture. Incubated for seven days at 25 °C on MEA agar. The round colonies show the mustard-yellow mycelium typical of the genus Talaromyces (fringed by a white margin) at the edge, which turns a strong blue color as the colony ages and spore production increases. Orange aerial mycelium forms in the center of the colony.
Picture 1b) Talaromyces islandicus from pure culture. Incubated for seven days at 25 °C on DG18 agar. The colony is lined with a thin margin of white mycelium followed by mustard yellow to orange mycelium with initial spore production. The center of the colony is also stained blue by increasing spore production. Aerial mycelium is sparsely formed at best.
Although the representatives of the genus Talaromyces are regularly detected in interiors, in the routine analysis of interior-relevant molds, nevertheless, often no clear demarcation from the genus Penicillium takes place. Therefore, the genera Penicillium and Talaromyces are often summed up together. With 250 - 290 (depending on counting of subspecies) known species, it is also not a small genus. Although taxonomically incorrect, this is common practice based on the available morphological data on the commonly used DG18 and MEA plates. This is based on the fact that few data on indoor relevance and physiological preferences are available for the genus Talaromyces, which is why closer differentiation of this mold genus increases taxonomic precision but rarely allows a better assessment of the mold cause.
When examining particle traces (total spore measurements) or adhesive film preparations, the spores of the genus Talaromyces are assigned to the spore type Aspergillus / Penicillium. A doubtless assignment is not possible without spore carriers. The spore type subsumes a number of mold genera whose spores can be described as small and round. The characteristics in direct microscopic methods are often insufficient to determine the genus beyond doubt.
According to TRBA 460, the mold species Talaromyces islandicus is classified in biological risk group 1. Thus, there is no known increased risk of infection to humans. In line with this, no entries for Talaromyces islandicus 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.
Talaromyces islandicus on self-produced creatinine sucrose agar
Talaromyces islandicus on self-produced creatinine sucrose agar (CREA). This medium is not used in routine analysis of indoor relevant molds. In interlaboratory studies, it serves as another tracer for species identification of molds. CREA plates are dark purple in color. To enhance nutrient uptake, some mold species can selectively acidify the surrounding culture medium, causing CREA plates to turn yellow. Due to the increased osmotic pressure and varying pH values, molds often grow more slowly on CREA plates.
Talaromyces islandicus on self-produced CYA
Talaromyces islandicus on self-produced Czapek Yeast Extract Agar (CYA). CYA is shown to be another medium that is not normally used in routine analysis of indoor-relevant molds. In this case, sodium nitrate is the only nitrogen source and sucrose is the only sugar source. According to the literature, this combination should favor the growth of heat-resistant (thermophilic) molds. In addition, chlamydospores are formed more frequently on CYA.
Scanning electron micrograph sputtered with gold at 4192x magnification
Scanning electron micrograph of Talaromyces islandicus sputtered with gold at 4192x magnification. Both the stalk of the spore carrier and the spores are smooth and show no discernible ornamentation from. The dents and deformations of the spore carrier and spores are preparation artifacts due to the vacuum formed. The genus Talaromyces forms the spores on phialides, which in turn sit on so-called metulae. These metulae form intermediate pieces between spore carrier and phialides. Characteristically, the length ratio of metulae to phialides within the genus Talaromyces is 1:1.