Penicillin comes from a mold belonging to the genus Penicillium. The antibiotic penicillin is naturally produced by certain species of this mold, principally Penicillium chrysogenum and Penicillium rubens (formerly known as P. notatum)

. The discovery was made in 1928 by Alexander Fleming, who observed that the Penicillium mold inhibited bacterial growth. He named the active antibacterial substance "penicillin" after the mold genus

. For mass production, scientists learned to cultivate Penicillium molds in deep fermentation tanks, using specific nutrients like sugars and corn steep liquor to boost penicillin yield

. The strain used for large-scale production, P. chrysogenum , was found to produce significantly more penicillin than Fleming’s original strain

. Thus, penicillin is derived from the natural metabolic products of Penicillium molds, grown under controlled fermentation conditions to produce the antibiotic for medical use.