Meet the Penicilliums

Penicillium is a genus of Fungi. I may have used the word 'mold' a few times - this is simply a term for a rapidly growing and asexually reproducing fungus. Because fungi were classified on the basis of the structures involved in sexual reproduction there used to be a group of Fungi, known as the Fungi imperfecti, that could not be classified because no-one had ever observed them to undergo sexual reproduction. This is less of a problem these days since we can look at their DNA and we understand more about how and why fungi switch from asexual to sexual reproduction.

Although the genus Penicillium is not particularly large it contains a surprisingly wide variety of both useful and nuisance species.

Good guys

• Penicillium chrysogenum (was notatum) produces Penicillin
  
• Penicillium camemberti, used, you guessed it, to produce Camembert (and Brie) cheese. Antibiotic substances produced by the fungus help keep bacteria from consuming the cheese.
  
• Penicillium glaucum used to produce a varity of cheeses including Gorgonzola and Stilton.
• Penicillium roqueforti used to produce Roquefort and other blue cheeses.

Bad guys

• Penicillium italicum and digitatum these are the ones you'll find on your citrus, and are both seen on the orange above (the smaller, bluer colony is italicum). Like many Penicillium species they grow well at cool temperatures and will easily grow in your fridge.
  
• Penicillium expansum causes a soft brown 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. Although patulin has strong antibiotic properties there are studies showing it has toxic effects on all cells (both human and bacterial) and there are therefore usually restrictions on how much patulin apple juice can contain.
• Penicillium marneffei is the only known species of Penicillium that can infect humans. It has recently been seen in AIDS patients in SE Asia with a fairly alarming incidence. 10% of patients in Hong Kong get Penicillosis (the disease caused by Penicillium marneffei).