Penicillium citreonigrum Dierckx 1901 - The story of the unplanned fungus
Taxonomy:
The mold species Penicillium citreonigrum was first described in 1901 in the “Annales de la Société Scientifique de Bruxelles” by Dierckx (R.P.). To this day, and significantly in the period before the Internet, the species Penicillium citreonigrum has been described under four other species names. The different names were subsumed under the first technical description in the course of the molecular processing and recording of species, which is why the name Penicillium citreonigrum is officially listed in the databases today (see mykobank.org as of 12.2024).
Picture 1) Back of a Penicillium citreonigrum pure culture on CYA agar (Czapek Yeast Agar). Incubated for seven days at 25 °C. This species forms a bright yellow coloration with a slightly dark center on the CYA agar, which is relevant for identification. The agar is not discolored or deformed by the mold growth.
Picture 2) Front of a Penicillium citreonigrum pure culture on CYA agar. Incubated for seven days at 25 °C. Pale gray-blue colonies can be seen, which can reach a diameter of 1.5 to about 2.5 cm. The colony edge (younger areas) is distinguished by its white coloration, as few conidiospores are still being formed here. Older areas of the colony (center), on the other hand, show the production of yellowish lipid droplets and yellowish aerial mycelium. Overall, the colony grows flat on the culture medium and shows a slightly raised center.
Picture 3) Front of a Penicillium citreonigrum pure culture on CREA agar (Creatine Sucrose Agar). The CREA agar used for species identification shows a purple coloration in a pH-neutral state. As can be seen in the pure culture of Penicillium citreonigrum, acid production leads to a color change. The agar becomes increasingly yellow. In the example shown, it is clear that Penicillium citreonigrum shows moderate growth and weak acid production on CREA agar compared to CYA agar.
Routine analysis:
As Penicillium citreonigrum is not described as having an increased water requirement, the species is not one of the recognized indoor humidity indicators (UBA mould guidelines as of 12.2024 and other specialist literature). As indicated in picture 1 - 3, characteristics that are relevant for reliable morphological determination can only be recorded if laboratories go to considerable extra effort with additional culture media (in addition to the usual identification via MEA and DG18). This is not possible in routine analysis, primarily due to time constraints, which is why the species Penicillium citreonigrum is generally subsumed with other species in the genus Penicillium. Molecular data, for example from ITS rDNA, suggest that there are cryptic species, depending on the species concept applied (mycobank.org as of 12.2024). Cryptic species can only be differentiated using molecular data and have no clearly separable morphological or physiological characteristics.
Medical importance:
According to TRBA 460 (2016:07), the mold Penicillium citreonigrum is classified in biological risk group 1. This means there is no increased risk of infection for humans. In line with this, the current Altas of clinical fungi (4th Edition 2020) does not describe any entries for Penicillium citreonigrum and therefore no known infections in humans. Specific information on the aW value and thus the expected water demand is not known. Sensitizing effects cannot be excluded due to the high spore production, especially with increased exposure.
Penicillium citreonigrum - and the story of the unplanned fungus
As described, Penicillium citreonigrum is neither a particularly relevant fungus in the detection of moisture damage, nor is it a medically relevant fungus. Nevertheless, the species is known in the scientific world due to two significant events, which is why we want to tell the story of the “unplanned fungus” in this context.
On the one hand, this species appears in the “1000 Fungal Genomes” project, but not because it is so relevant and interesting for science, but rather because the genome of Penicillium citreonigrum was sequenced by mistake, although it was actually a Cerinomyces species that was targeted. Unfortunately, the corresponding sample was so heavily contaminated with Penicillium citreonigrum that nothing of the target species Cerinomyces crustulinus could be detected.
On the other hand, Penicillium citreonigrum has played a tragic and far-reaching role in attempts to improve the nutritional situation of people in poorer countries. Even before the Second World War (around 1937), it was discovered in Japan that incorrectly stored rice can be infected by Penicillium citreonigrum. The mycotoxin citreoviridin produced by the fungus not only causes a yellowish discoloration of the rice, but also makes it inedible (it is also suspected that some fatal food-borne infections can be traced back to the “yellow rice”). This problem could only be solved by a comprehensive hygienic improvement in the storage conditions of rice and today plays a comparatively minor role in food production. Nevertheless, yellow rice has since been considered inedible and harmful to health in most Asian countries. Unfortunately, “golden rice”, a genetically modified rice grain with a greatly increased vitamin A content, is similar in color to the yellow rice infected by Penicillium citreonigrum. Just the rough visual similarity between the yellow rice and the golden rice leads to a strong rejection of the golden rice, which was actually intended to improve the vitamin A supply in the Asian region.
The two cases described show where a single unplanned fungus can seriously disrupt scientists' plans.
Front pure culture on MEA agar
Incubated for seven days at 25 °C. Both the yellowish lipid droplets and the (faint) yellow aerial mycelium can be seen in the center of the colony. The growth area of the colony (outer edge on the left in the picture) shows individual mycelial strands without conidiospores, which therefore still stand out in white.
Light microscope image at 400x magnification
The picture shows a monoverticillate spore carrier with a smaller lateral branch. The style is smooth and shows a segmentation just below the branching (faintly visible) under the transmitted light microscope. The spore-bearers of Penicillium citreonigrum are comparatively short at 70 - 120µm and rather delicate compared to other Penicillium species. At the end of the spore carrier, 2 - 4 phialides (up to 8 phialides can occur) with short spore chains can be seen in the picture shown.
Scanning electron micrograph sputtered with gold at about 9000x magnification
Spores, spore carriers and phialides are compressed by the sputtering process and the vacuum (preparation artifact). Normally, the structures mentioned are turgescent and round, without clear deformations. None of the three structures mentioned show any significant surface structure.
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