Where Is Penicillium Mold Commonly Found?

Penicillium mold turns up in an extraordinary range of places, from kitchen fruit bowls and household dust to Arctic glaciers and deep-sea sediments. With over 300 recognized species, Penicillium is one of the most widespread fungal genera on Earth. Its spores drift through outdoor and indoor air almost everywhere humans live, and its colonies thrive on materials as varied as damp drywall, rotting citrus, stored grain, and aged cheese. Understanding where Penicillium shows up helps explain why it matters so much to food safety, indoor air quality, medicine, and even environmental cleanup.

Indoor Environments and Household Dust

If you have ever opened a forgotten container of leftovers and found a fuzzy blue-green coating, you have probably met Penicillium. Inside homes and buildings, it is one of the dominant mold genera. A study of the invisible fungal community in domestic air and dust found a core indoor mycobiome dominated by Penicillium along with Cladosporium, Aspergillus, and Alternaria. Elevated levels of airborne Penicillium spores were specifically tied to indoor sources like rotting food.¹

Beyond the kitchen, Penicillium colonizes building materials themselves. On wood, wood composites, and starch-containing materials, mold growth can begin at around 78 percent relative humidity at room temperature. Gypsum board (the material inside most interior walls) supports growth at about 86 percent relative humidity. When researchers tested what actually grew on these substrates, Penicillium and Aspergillus species dominated almost exclusively.² This is why moisture problems in a building so reliably produce Penicillium colonies. A leaky pipe behind a wall, condensation in a poorly ventilated bathroom, or a damp basement can all create the humidity conditions the mold needs.

Penicillium is also used as an indicator when investigating suspected moisture damage in homes. Concentrations of Penicillium, Aspergillus, and Eurotium in fine house dust can help predict hidden water damage behind walls or under floors.³ In other words, if unusually high levels of these molds turn up in dust samples, there is likely a moisture problem somewhere in the structure.

Fruit, Grain, and Other Food

Penicillium is responsible for some of the most economically damaging post-harvest diseases in agriculture. If you have ever seen a lemon or orange develop a spreading patch of green or blue fuzz in the fruit bowl, that is almost certainly a Penicillium species at work. Green mold (Penicillium digitatum) and blue mold (Penicillium italicum) are considered among the most costly post-harvest diseases of citrus worldwide.⁴

Apples face their own Penicillium nemesis. Blue mold caused by Penicillium expansum is a major threat to the global pome fruit industry, causing losses worth millions of dollars each year and reducing fruit quality.⁵ The problem extends into storage. In cold-stored apples, P. expansum and P. griseofulvum can grow and produce the mycotoxin patulin. Researchers found that apples kept at refrigerator temperature remained free of detectable patulin for about 27 days, but once those same apples were moved to room temperature for even three days, both the visible rot and patulin levels jumped sharply.⁶ That window between cold storage and processing is critical for food safety in juice production.

Stored grains and grain products are another common habitat. Toxin-producing molds frequently contaminate grain kept under poor storage conditions, and several Penicillium species are among the culprits.⁷ Cereal crops, beans, dried fruits, herbs, and spices can all harbor Penicillium if humidity and temperature favor fungal growth.

Cheese and Intentional Food Uses

Not every encounter with Penicillium on food is unwelcome. Penicillium roqueforti is the fungus responsible for the blue veins in cheeses like Roquefort, Gorgonzola, and Stilton. Different populations of P. roqueforti have adapted to distinct cheese-making traditions. Researchers have identified that the non-Roquefort population of P. roqueforti shows higher tolerance to lactic acid, a compound produced during fermentation that normally inhibits fungal growth. This population could keep growing at lactic acid concentrations where other populations stalled, suggesting it has specifically adapted to the cheese environment.⁸

The relationship between P. roqueforti and food safety is more nuanced than “blue cheese mold = harmless.” The fungus can produce mycotoxins of its own, including roquefortine C and mycophenolic acid.⁹ In commercially produced blue cheese, the levels of these toxins are generally low and considered safe for most people. But researchers have recently used gene-editing techniques to develop P. roqueforti strains that cannot produce either toxin, opening the door to toxin-free blue cheese.

Penicillium camemberti plays a similar role in soft-ripened cheeses like Camembert and Brie, where the white rind you see on the outside is a carefully managed mat of the fungus. The same genus responsible for ruining your fruit can, under controlled conditions, create some of the world’s most prized foods.

Soil, Plant Roots, and Living Plants

Outdoors, soil is one of Penicillium’s primary habitats. A survey of rhizosphere soil (the soil clinging to plant roots) across forests and coastal sand dunes in South Korea identified 26 Penicillium species from just 64 isolated strains. Forests yielded about 15 species and sand dunes about 12, with most species appearing at only one type of site.¹⁰ The diversity is striking even in a geographically limited area, and it varies depending on the plant species growing above.

Some Penicillium species do more than just live near plants. They actually live inside them as endophytes, colonizing the internal tissues of roots, stems, and leaves without causing disease. These endophytic strains have been characterized as plant growth promoters, enzyme producers, and stress defenders that help their host plants tolerate drought, salinity, and other harsh conditions.¹¹ One example is Penicillium citrinum, a drought-tolerant endophyte isolated from wheat leaves. When used as a biological inoculant, it enhanced the wheat plant’s physiological responses under drought stress, producing plant growth hormones and helping the plant access phosphate and zinc.¹²

Another species, Penicillium olsonii, was isolated from the rhizosphere of a salt-tolerant grass growing in a saline flat. When tested in a hydroponic system, it promoted plant growth and enhanced salt stress tolerance so effectively that researchers proposed it could reduce the need for chemical fertilizers.¹³ These beneficial roles are part of why agricultural researchers pay close attention to soil Penicillium.

Airborne Spores

Penicillium spreads through tiny spores that ride air currents. Each colony can produce millions of these microscopic particles, which is why the genus shows up in air samples almost everywhere researchers look. A six-year air monitoring study in a temperate European city found that Penicillium and Aspergillus spores were present in high numbers during late summer and early autumn, peaking from August through November.¹⁴ The seasonal pattern tracks with conditions that favor fungal sporulation: warmth combined with decaying vegetation in late summer, then a secondary surge as autumn dampness sets in.

Indoors, Penicillium spore counts do not necessarily follow outdoor seasons. A home with active mold growth on damp materials or spoiling food can have elevated indoor Penicillium year-round. This is partly why indoor air quality assessments specifically look for genera like Penicillium and Aspergillus whose presence at elevated levels signals an indoor moisture source rather than just normal infiltration from outdoor air.

Extreme and Unexpected Habitats

Penicillium does not just stick to comfortable, human-adjacent environments. It turns up in places that seem almost hostile to life. Researchers studying Arctic glaciers on Svalbard, Norway, found remarkably high concentrations of Penicillium in sediment-rich subglacial ice, with counts reaching up to 9,000 colony-forming units per liter. The dominant species was P. crustosum, making up roughly half of all isolates, but more than 20 additional Penicillium species were also recovered.¹⁵ These organisms are not necessarily thriving at subzero temperatures, but they survive in a dormant state and can resume growth when conditions warm.

At the other end of the spectrum, two novel Penicillium species were isolated from subseafloor sediments in the South Pacific Gyre, one of the most nutrient-poor ocean regions on the planet. These deep-sea strains contained genes for breaking down tough carbon compounds, suggesting they play a role in carbon cycling even in extreme deep-ocean environments.¹⁶

Hypersaline environments also support a surprising diversity of Penicillium. A survey of salt lakes and other high-salinity waters identified 27 Penicillium species, all of which could grow on media containing 17 percent sodium chloride, though most grew better without such extreme salt levels.¹⁷ A strain of P. chrysogenum isolated from a Bulgarian salt-production lake demonstrated even more dramatic salt tolerance, growing on media with up to 20 percent sodium chloride, which qualifies it as moderately halotolerant.¹⁸

Penicillium species also associate with insects and their habitats. A study of stingless bees in Brazil recovered multiple Penicillium and Talaromyces species from honey, pollen, and the inside of bee nests.¹⁹ The bees likely pick up the spores during foraging and carry them back, making the nest a secondary habitat for the fungus.

Mycotoxins and Food Safety Concerns

Several Penicillium species produce mycotoxins, natural toxic compounds that can contaminate food and animal feed. Patulin, discussed earlier in the context of apples, is one of the best known. Another is citrinin, a mycotoxin produced by certain species of Penicillium, Aspergillus, and Monascus. Citrinin can contaminate beans, fruits, juices, herbs, spices, and dairy products, and it can enter the food chain at any stage from pre-harvest through storage.²⁰

Ochratoxin A is yet another Penicillium-associated mycotoxin. Citrinin and ochratoxin A commonly co-occur in livestock feed, posing a combined threat to animal health and productivity.²¹ The practical takeaway is that the mere presence of Penicillium on food or feed does not automatically mean danger, since many species are harmless or even useful, but certain species under certain conditions are genuine safety hazards. Proper storage temperature and humidity control remain the most effective defenses.

Apple cultivars differ in their susceptibility to patulin accumulation. In one study of traditional and conventional apple varieties, some cultivars accumulated very high concentrations of patulin after infection with P. expansum, while in others the toxin was undetectable. The variation correlated partly with the apple’s polyphenol content, with certain phenolic compounds appearing to influence how much patulin the fungus produced.²² Breeding or selecting resistant varieties could be a practical long-term strategy for reducing patulin in apple-derived products.

Health Effects of Indoor Exposure

For most healthy people, the Penicillium spores floating through everyday air are a nonissue. Your immune system clears them without you ever knowing they were there. But for people with asthma or allergic sensitivities, indoor Penicillium exposure can be a real trigger. A systematic review of longitudinal studies found a consistent, albeit modest, association between mold exposure in asthmatic children, particularly exposure to Penicillium, and worsened respiratory outcomes including more symptom days and more unplanned hospital visits.²³

The numbers tell a clearer story. In one study, the presence of any Penicillium in the home roughly doubled the odds that an asthmatic child would have high peak expiratory flow variability, a measure of how much their breathing fluctuated day to day.²⁴ Another study found that children who were both sensitized and exposed to Penicillium had about twice the risk of wheeze, persistent cough, and higher asthma severity scores compared to those who were not sensitized or not exposed.²⁵

Invasive Penicillium infections are rare. In people with healthy immune systems, serious Penicillium disease essentially does not occur. In severely immunocompromised patients, however, Penicillium species are increasingly recognized as opportunistic pathogens, with infections typically limited to the respiratory tract.²⁶ The globally relevant pathogenic species is Talaromyces marneffei (formerly classified as Penicillium marneffei), which causes disseminated infection in immunocompromised individuals, primarily in Southeast Asia.

The Penicillin Connection and Industrial Uses

Penicillium’s most famous contribution to human welfare is, of course, penicillin. The species historically credited with Alexander Fleming’s 1928 discovery is Penicillium chrysogenum, which has been used for industrial penicillin production since the 1940s.²⁷ Molecular work has since reclassified Fleming’s original strain as Penicillium rubens, a closely related species that had been lumped together with P. chrysogenum for decades.²⁸ The two look nearly identical under a microscope, which explains the confusion.

Modern strains used in pharmaceutical manufacturing have been selectively bred and genetically tweaked over many generations to produce penicillin at concentrations orders of magnitude higher than wild strains. P. chrysogenum also serves as a model organism in fungal biotechnology more broadly, contributing to the production of other beta-lactam antibiotics and various industrial enzymes.

Plastic Degradation and Emerging Roles

One of the more surprising recent findings about Penicillium involves electronic waste. Researchers tested Penicillium brevicompactum against microplastics derived from printed circuit boards, a common component of e-waste. Over just 14 days, the fungus reduced the mass of these microplastics by up to 75 percent and caused visible fragmentation, chain-splitting, and chemical oxidation of the plastic surfaces.²⁹ This is early-stage research, not a ready-made solution to the plastic pollution crisis, but it illustrates how the same genus that spoils your bread could eventually help clean up contaminated environments. The ability of various Penicillium species to break down tough organic and synthetic materials is tied to the rich arsenal of enzymes they produce, enzymes that evolved to decompose dead plant matter in soil and now turn out to work on human-made polymers as well.