Penicillium is a genus of fungi, growing as green, blue or white molds on decaying food, especially fruit.

The name Penicillium comes from the Latin word for brush, penicillus, based on the brush-like shape of the fruiting (spore-bearing) structures as seen under a microscope.

For the most part, Penicillium species like low temperatures. It is a versatile and opportunistic fungus than employs a wide range of enzymes to attack a host of organic foodstuffs. Eating infected food can be dangerous to animals and humans alike, as the fungi can produce dangerous toxins (mycotoxins), some of which are carcinogenic, and others that can cause serious deleterious effects.

Penicillium isn't all bad, however. Some species are used to produce and ferment cheese. Nor should we forget P. notatum and P. chrysogenum, from which we can produce the antibiotic Penicillin.

There are over 230 species of Penicillium, but some of the best known are:

P. italicum - blueish, most commonly found on citrus fruit.

P. digitatum - olive-green, also fond of citrus fruit.

P. roqueforti - blue, used in the production of Roquefort cheese.

P. gorgonzola - bluish-green, used in cheese production (e.g. gorgonzola).

P. expansum - blue/white, often found on inappropriately stored apples.

P. notatum - white, used to produce Penicillin, which the mold secretes as it grows.

P. chrysogenum - similar to notatum.

P. glaucum - blue, used in production of cheese.

P. wortmanii - used to produce wortmannin.