Mold of the month July: Penicillium griseofulvum
Penicillium griseofulvum was first described morphologically by Dierckx in 1901 and recognized as a distinct species within the genus Penicillium. The morphological identification of Penicillium griseofulvum is comparatively simple as it is one of the few interior Penicillium species to form tertiary and quarterverticillate spore carriers. That means, from one main axis branch up to three lateral axes, which end in terminal metulae and phialides.
Picture left: Penicillium griseofulvum from pure culture. Incubated for seven days at 25°C on CYA agar. The circular colony often exhibits a white bulging rim of sterile mycelium. The center of the colonies is distinguished by massive spore production and the associated grayish coloration. In older colonies (>5 days) exudate droplets are often formed. These can be colorless to slightly yellowish. The exact composition of these exudate droplets is unknown, but presumably they also contain sugar sources in addition to mycotoxins.
Picture right: Penicillium griseofulvum from pure culture. Incubated for seven days at 25°C on MEA agar. The colony also shows a white border of sterile mycelium and is clearly darker gray in the center. Production of exudate droplets takes place to a lesser extent. In the center of the colony, sporadic aerial mycelium is formed (lighter areas).
In the laboratory, more heavily contaminated Penicillium griseofulvum samples can be recognized by a very intense fruity odor, which is easily perceived even with closed agar plates. The odor is reminiscent of multivitamin juice. Detailed measured values for water demand (aW value) are not known (mold guide from the Federal Environment Agency 2017), but the frequent detection of Penicillium griseofulvum in dried foodstuffs suggests a low water demand and thus more likely a xerophilic species (Food and Indoor Fungi Sec. Ed. 2019).
Penicillium griseofulvum is widespread throughout the world even though this species is not one of the most common in indoor environments (Domsch et. al. 2007 Sec. Ed.). Penicillium griseofulvum infestation in seeds can lead to a significant reduction in the germination capacity of plant seeds. A broad cocktail of mycotoxins including patulin (Food and Indoor Fungi Sec. Ed. 2019) is probably to blame. However, the best-known mycotoxin of Penicillium griseofulvum is griseofulvin, which was discovered in 1936 and long used as an antimycotic in human medicine.
With a maximum growth temperature of 35°C, Penicillium griseofulvum is not a thermotolerant species. According to the TRBA 460, this species is to be classified in biological risk group 1 and pathogenicity in humans has not yet been established (Atlas of Clinical Fungi 4th Ed. 2020).
At 400x magnification under the light microscope
Quarterveriticillate spore carriers of Penicillium griseofulvum at 400x magnification under the light microscope. From the main axis of the spore carrier, three successively shorter, phialid-bearing lateral axes branch off towards the top. The main axis ends in terminal phialides. All phialides are bottle-shaped (very short) and stand on metulae. Lateral axes are each just below a septum, which divide the main axis. The conidiospores are round and formed in long chains. Preparation stained with cotton blue.
At 5150X magnification under SEM
Spore carriers and mycelium of Penicillium griseofulvum sputtered with gold under vacuum at 5150X magnification under scanning electron microscope (SEM). Spore carriers with metulae, phialides and spore chains starting from mycelium. In the example shown terverticillate (with two lateral axes). Spores and spore carriers correspond to preparation artifacts caused by vacuum sputtering.