Penicillin is a specific mold, and not a common find in the kitchen when food goes moldy.
Controlled experiments are how it is cultured to make medicine, and it is not something the average, un-trained, non-chemist should try at home.
While relache is mostly correct, I would like to be a little more precise:
The genus Penicillium sp. is pretty widespread, so there is a chance that the mold growing in your food may be a penicillium. However, these molds not always produce penicillin, and most of the time they do produce toxins, so don't even think about consuming them.
To extract penicillin from mold, complicated chemical processes are needed.
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