Penicillium chrysogenum Thom (1910)
The name Alexander Fleming is likely familiar even to those without a background in biology. Fleming was the scientist who, by chance, discovered penicillin – one of the most significant antibiotics in human history. It’s no exaggeration to say that medicine is often divided into two eras: before and after the discovery and commercial use of antibiotics. But what does this have to do with Penicillium chrysogenum, the species we’re focusing on here?
The Brush Behind Fleming’s Breakthrough
In September 1928, during what became his “failed” experiment, Alexander Fleming was actually studying Staphylococcus bacteria. Over time, some of his bacterial cultures became contaminated. A mold had found its way onto the agar plates, and upon closer examination, it produced a remarkable effect: the otherwise vigorous bacterial growth was inhibited in the immediate vicinity of the mold. Even more surprisingly, areas already colonized by bacteria showed a reduction in growth near the mold. The fungus was evidently capable of suppressing bacterial development, a phenomenon that was virtually unknown and certainly not understood by the scientific community at that time.
The contamination originated from a fungus then known as Penicillium notatum, which is now recognized as a synonym for Penicillium chrysogenum. This mold belongs to the so-called brush molds, named for the characteristic brush-like shape of their spore-bearing structures. In reference to the fungal genus, the newly discovered antibacterial substance was named “penicillin.” However, penicillin was not officially approved as a medical product until 1942, during the pressures of World War II.
For the record, Fleming was not the first to observe the antibacterial effects of penicillin. Reports of such activity date back to 1870, but at that time, science lacked the tools to apply this knowledge effectively (see www.dpma.de; as of August 2025). Nearly a century later, Penicillium chrysogenum remains one of the key model organisms in mycology, widely used in research on secondary metabolites and genomics.
Of the roughly 5,500 known antibiotics (as of August 2025), about 160 are officially approved as medical products. Even today, important substances such as griseofulvin, fusidic acid, and cephalosporins are produced by molds. This knowledge, along with the striking microscopy images of Penicillium chrysogenum, may help us appreciate this so-called “indoor contaminant” not only for its negative effects but also for its significant scientific and medical contributions.
A Brief Excursion into the World of Microorganisms
Why do molds and bacteria often produce substances that inhibit or destroy other microorganisms? Simply put, these are competitive strategies, mechanisms that help them secure limited resources such as nutrients, space, and water. Due to their limited mobility, bacteria, fungi, and even plants rely on chemical interactions with their environment to outcompete rivals.
Among bacteria, an important distinction can be made, one that’s also relevant to modern cleaning products. Bacteriostatic substances inhibit bacterial growth and may kill cells in extreme cases, whereas bacteriolytic substances always kill bacteria and also dissolve their cell structures. When treating mold damage indoors, a lytic product is often preferred, as it helps remove allergenic spores. However, caution is needed: cell breakdown can lead to increased release of mycotoxins.
Picture 1) Front view of a Penicillium chrysogenum pure culture on MEA agar incubated for eight days at 25 °C.
Morphological identification is possible using direct microscopy, based on the presence of ter- or quaterverticillate conidiophores in combination with relatively large colonies and the light yellow exudates visible in the image. Depending on the medium, these exudates may appear more intensely yellow. All three colonies exhibit the typical white margin of sterile mycelium, where spore production has not yet begun due to age. It is interesting to observe how the three colonies interact with each other. The concentric growth pattern is disrupted where the colonies meet, likely due to growth inhibition caused by the released exudates, which may also affect other molds.
Picture 2) Front view of a Penicillium chrysogenum pure culture incubated for eight days on DG18 agar.
The colony is similar in size to that grown on MEA but shows fewer exudate droplets and develops small tufts of aerial mycelium in the center. Overall, the colony has a velvety appearance. Despite the conidiophores reaching lengths of up to 500 µm, the colony does not extend significantly above the agar surface. The mutual inhibition observed on MEA (presumably caused by exudates) is only weakly expressed on DG18 agar.
Taxonomy:
Since the first officially recognized description by Thom in 1910, no fewer than eight additional descriptions of Penicillium chrysogenum have been published, along with numerous morphological and orthographic variants that have made its taxonomy even more complex (Source: Mycobank.org, August 2025). In the third edition of Food and Indoor Fungi, fourteen species descriptions are now subsumed under Penicillium chrysogenum.
Of particular note are three 1901 publications in Annales de la Société scientifique de Bruxelles, in which the same author, Dierckx, described what is now known as Penicillium chrysogenum three times under three different names. Whether this was a misunderstanding of the species’ phenotypic variability or an attempt to boost publication count remains unclear. Before the advent of molecular taxonomy, high-resolution microscopy, and the Internet (which has made literature more accessible), keeping track of described mold species was challenging. The sheer number of fourteen separate species descriptions alone indicates the frequency and scientific importance of Penicillium chrysogenum. This is further supported by the approximately 78 strains and 85 genomes of this species deposited in the CBS (Centraalbureau voor Schimmelcultures) database (Source: Mycobank.org, August 2025).
Routine analysis:
In routine laboratory diagnostics, Penicillium chrysogenum is recognized as a reliable moisture indicator. It grows on damp materials and has a reported water activity (aW) range of 0.78–0.81 (Source: UBA Mold Guide 2024). Its ability to tolerate both saline (halotolerant) and cold (psychrotolerant) conditions, as well as to grow on almost any carbohydrate-containing substrate, demonstrates remarkable ecological versatility (Food and Indoor Fungi, 3rd Edition). The fact that Penicillium chrysogenum can even colonize dried foods shows that it can thrive under both moist and dry environmental conditions. This adaptability makes it a resilient indoor mold species that can persist even in building materials with fluctuating moisture levels. Under laboratory conditions, it grows relatively quickly and produces abundant spores on its highly branched ter- to quaterverticillate conidiophores (up to 500 µm long). The extended conidiophores likely aid wind dispersal of spores, while their complex branching provides more surface area for spore production.
In short, Penicillium chrysogenum is a highly adaptable and prolific mold. Detecting it in laboratory samples is critical, as its presence can influence the assessment of indoor mold contamination.
In addition to indicating moisture-damaged materials (such as plaster, lime, cement, or wood), contaminated soil, or spoiled foods, this mold plays another key role indoors. It produces metabolic by-products such as oxalic, citric, and gluconic acids, which are secreted into the surrounding environment. In high concentrations, these acids can cause permanent discoloration of building materials by chemically altering or destroying pigments, particularly iron oxides used in paints and coatings. As a result, lightening of materials can remain visible even after the mold itself has been removed.
On lime-based plasters, the organic acids can also cause loss of structural strength, leading to more serious material damage.
Medical relevance:
At first glance, the medical relevance of Penicillium chrysogenum might appear alarming, as isolated fatal infections have been reported. However, these are extremely rare cases, documented over several decades and always associated with pre-existing conditions.
According to the Atlas of Clinical Fungi (4th Edition, 2020), occasional infections of the skin, brain, lungs, and other organs have been documented, though such occurrences are exceedingly uncommon. Only a few strains can grow weakly at 37 °C, supporting the general view that this species poses minimal health risk.
Overall, Penicillium chrysogenum and its variants are classified as biosafety level 1 organisms, meaning they are not considered hazardous under normal conditions. The only noted caution is the subcategory “A,” which indicates that allergenic effects may occur, a factor to consider when assessing indoor mold contamination (Source: TRBA 460, July 2016).
Microscopic image at 1000x magnification
The three-dimensional structure of the conidiophore makes it difficult to capture an image with a uniform focal plane in which multiple sections appear sharp. The depth of field of the chosen objective lens is limited, in this case to just a few nanometers.
In the center of the image, extending from the right, a terverticillate conidiophore with individual septa is shown; from below, a biverticillate conidiophore enters the field of view. Conidiophores of Penicillium chrysogenum are typically terverticillate but may also be quaterverticillate or, more rarely, even more complex.
What does “x-verticillate” mean? This term describes, starting from the base of the conidiophore at the mycelial strand (often via a foot cell), the number of spore-bearing branches that arise at each level.
In the specific case of a terverticillate conidiophore, the first branch extends upward, followed by a second branching downward and the termination of the main axis. In P. chrysogenum, the side branches are similar in length and number of metulae to the main axis. Spores are produced basipetally by phialides that arise from the metulae. Typically, two to three phialides are found on each metula, and the metulae are longer than the phialides. Spores are formed in long chains and are nearly spherical, up to 4 µm in diameter. The axis along the spore chain is usually slightly shorter than the perpendicular axis. P. chrysogenum responds well to the staining used (cotton blue + Shear’s solution), resulting in a uniform and intense blue coloration.
Scanning electron microscope image with gold sputtering at approximately 8062x magnification
The scale bar on the right represents 20 µm. In the center, a terverticillate conidiophore is visible. Neither the spores nor the conidiophores exhibit significant surface ornamentation. The spores are produced in long chains. Sample preparation must be done carefully, as the chains easily fragment into smaller pieces. Consequently, spore chains observed in particle measurements or tape-lift samples can be considered evidence of local spore formation near the sampling site. The depressions on the spores and the flattened mycelial strands are preparation artifacts caused by vacuum conditions and sputter-coating.
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