Penicillium chrysogenum

Penicillium chrysogenum produces penicillin—the antibiotic that launched the antibiotic era. While Alexander Fleming's original discovery was from P. rubens (then called P. notatum), industrial penicillin production developed using P. chrysogenum strains selected for higher yields. The parallel between fungal penicillin and bacterial streptomycin production demonstrates convergent evolution: both fungi and bacteria independently evolved to produce antimicrobial compounds for ecological competition.

Industrial penicillin production required enormous strain improvement. Original isolates produced tiny amounts; modern industrial strains produce thousands of times more. This improvement came through classical strain improvement—repeated rounds of mutation and selection—before genomic tools existed. The resulting strains are among the most heavily modified microorganisms in industrial use, though all modifications arose through selection on random mutations rather than rational engineering.

P. chrysogenum illustrates how natural products become industrial commodities. The molecule that evolved for fungal ecological competition became a manufactured pharmaceutical through strain improvement, fermentation optimization, and downstream processing development. Understanding the biosynthetic pathway eventually enabled semisynthetic penicillins—chemically modified versions with improved properties. The penicillin story demonstrates the full arc from ecological discovery through industrial production to derivative development.