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Production

Penicillin is produced by the fermentation of various types of sugar by the fungus Penicillium rubens. The fermentation process produces penicillin as a secondary metabolite when the growth of the fungus is inhibited by stress. The biosynthetic pathway outlined below experiences feedback inhibition involving the by-product l -lysine inhibiting the enzyme homocitrate synthase.

The Penicillium cells are grown using a technique called fed-batch culture, in which the cells are constantly subjected to stress, which is required for induction of penicillin production. While the usage of glucose as a carbon source represses penicillin biosynthesis enzymes, lactose does not exert any effect and alkaline pH levels override this regulation. Excess phosphate, available oxygen, and usage of ammonium as a nitrogen source repress penicillin production, while methionine can act as a sole nitrogen/sulfur source with stimulating effects.

The biotechnological method of directed evolution has been applied to produce by mutation a large number of Penicillium strains. These techniques include error-prone PCR, DNA shuffling, ITCHY, and strand-overlap PCR.

Semisynthetic penicillins are prepared to start from the penicillin nucleus 6-APA.