Common Indoor Mold: Penicillium
Most people are familiar with Penicillium, even if they don’t realize it. Penicillium is a common indoor mold and can frequently be found in your kitchen, colonizing fruit or bread. If you’ve ever left an orange around too long only to come back to it and find a green fuzzy mold on it, you’ve seen Penicillium. On lab media (and food), Penicillium mold is fast growing, powdery in texture, and often green or blue-green in color (Fig. 1), although some species can be whitish, yellow, gray or even reddish.
There are over 200 species of Penicillium molds and this number is growing as scientists develop better tools to differentiate the species. Penicillium is one of the most frequently identified molds in environmental samples and is naturally found in temperate soils and decaying plant matter. Tens to hundreds of spores are produced on each brush-shaped spore head (called a conidiophore), so Penicillium spores are also very commonly found in air samples. These spores are often chained together and are very tiny, at only 2-4 µm in diameter. For reference, this is about 1/10,000 of an inch. Because the spores are so small and lightweight, they can remain airborne for a long time, making it easier for the mold to spread and to come into contact with humans.
Penicillium: A microscopic view
Under the microscope, Penicillium structures are difficult to see because they are nearly colorless (called hyaline). In the below figure on the left, you can barely discern spore-forming condiophores (Figure 2A). Fortunately, analysts have dyes that are absorbed by the fungal structures, making fungal identification much easier and quicker (Figure 2B). Penicillium mold consists of long fibrous strands called hyphae that can branch. At the terminal ends of the hyphae branches, sporulating heads form that look a lot like skeletal hands (called phialides). The spore heads can be simple or elaborately branched and produce long chains of round spores from the tips of the phialides.
Penicillium: In the Home
In indoor environments, Penicillium spores are one of the most frequently identified spore types. One study that examined over 12,000 air samples was able to detect Penicillium spores in up to 80% of indoor air samples and up to 77% of outdoor air samples (Ref 1). In indoor air, researchers estimated an average of 30 spores per cubic meter of air and 50 spores per cubic meter of air in outdoor environments. Penicillium is everywhere!
Besides causing food spoilage in the kitchen, Penicillium can take up a more undesirable residence in your home. With the right growth conditions, Penicillium can grow on damp building materials including ceiling tiles, walls and wallpaper, flooring, and carpeting. The image shows a sample of vinyl wallpaper our lab received that was covered with dark colored Penicillium colonies. Penicillium prefers cool to moderate temperatures and is unlikely to grow well when temperatures exceed 100F, making living indoors a very attractive option for this mold
Besides being a nuisance, Penicillium molds can have some severe health effects. Indoor molds, including Penicillium, have been associated with asthma and other respiratory symptoms (Ref 2). Penicillium species also can produce interesting secondary byproducts – from the helpful penicillin antibiotic to toxic metabolites called mycotoxins.