Penicillium italicum and Penicillium digitatum on Orange
The name Penicillium comes from penicillus = brush, and this is based on the brush-like appearance of the fruiting structures (above right) under the microscope. Penicillium typically produces these brush-like heads. The stalk is called the conidiophore. The conidiophore branches at the tip. At the end of each branchlet is a cluster of spore-producing cells called phialides. A chain of spores is formed from the tip of each phialide. The spore is called a conidium 0r phialsopore. The spores in Penicillium often contain blue or green pigments which give the colonies on foods and feeds their characteristic colour. It is the spores in the blue cheese that give the colour to the cheese. The spores are only a few microns in diameter. I wonder how many millions of spores are eaten in a serving of blue cheese. How would you figure it out? ( hint: need a haemocytometer).
This orange (above left) was inoculated with two species of Penicillium at the same time. The smaller bluish colony is Penicillium italicum. The larger olive-green colony is Penicillium digitatum. These are the two common species of Penicillium attacking citrus fruits. Millions of $$$$ worth of losses are caused each year by these fungi during storage or transit of citrus fruits. The fact that P. digitatum grows much faster doesn't necessarily mean that it is the more serious rot fungus. It depends on the temperature. As I recall this orange was incubated at room temperature (20C). If we had incubated at 5C (= refrigeration temperature) then perhaps P. italicum would be faster. Try it!
For the most part Penicillium species like the temperature on the cool side. Penicillium is a versatile, opportunistic fungus with an arsenal of useful enzymes at its disposal to attack a host of organic foodstuffs. So, you have probably already seen it a number of times on food left a little too long in the fridge. It is partial to bread, cheese, cold meats, old sandwiches, cereal products and a host of other things.
If you are a farmer and store your cereals in bins then Penicillium is a dangerous adversary. If the percentage moisture in your cereal is too high then Penicillium will happily destroy it. While it is growing in the feed, the fungus can produce dangerous toxins (mycotoxins) in the cereal residues than can cause serious deleterious effects in animals consuming contaminated feed.
Even fruit farmers can't escape the ravages of Penicillium. Penicillium expansum causes a soft rot of apples. The rotted part of the apple contains a mycotoxin called patulin produced by the fungus. If such apples are squeezed for juice then the patulin persists in the juice. So, apples with brown rot should never be used for apple juice. The mycotoxin patulin is a broad spectrum antibiotic and suppresses both gram positive and gram negative bacteria as well as other fungi. Penicillium species break down food products by external secretion of enzymes. The production of antibiotics will inhibit competition and protect the substrate for the exclusive use of the Penicillium. At least for a little while. Think about it!
Penicillium isn't all bad, however. On the plus side we have Penicillium roquefortii used in the manufacture of blue cheese. During the fermentation process the fungus imparts a pleasant tang to the final product. By the way, the blue in the blue-cheese is caused by the pigment in the spores (conidia) of the fungus. You are consuming spores by the million when you eat blue cheese. Hope it doesn't put you off! Also, let us not forget the contributions of Penicillium notatum and P. chrysogenum in the production of the antibiotic penicillin.