Penicillium notatum, contamination

Figure 1.14. The very petri dish that sparked the discovery of penicillin. The white blob at the bottom is a colony of Penicillium notatum contaminating a plate streaked with Staphylococcus aureus (small, circular colonies). The penicillin diffusing from the fungus radially into the agar has killed off the bacterial colonies in its vicinity.

One of the most important events in the history of antibiotics is the discovery and production of penicillin. In 1928, while investigating staphylococcus variants at St Mary s Hospital in London, Alexander Fleming observed that when a particular strain of mold, Penicillium notatum (named because the cells were pencil-shaped when viewed under a microscope), contaminated these cultures they underwent lysis. Fleming named the active substance penicillin. Unfortunately, because Fleming was such a poor public speaker, his public presentation of his seminal discovery went unheeded, and it took an additional 12 years before the potential of penicillin was realized. 

The original mold observed and preserved by Alexander Fleming was a strain of Penicillium notatum, a common laboratory contaminant. Later, cultures of Penicillium chrysogenum were found to be better producers of penicillin, and the present industrial strains have been derived from this species. The original strains produced the antibiotic only by surface fermentation methods and in very low yields. Improved media and productive strains under submerged aerobic fermentation conditions led to dramatic yield increases. Subsequent improvements, principally in culture selection and mutation, further improved yields, reaching 20-30 g/L. 

He initially thought that the mould was a potent strain of Penicillium rubrum, although it was subsequently shown to be Penicillium notatum. He screened a number of other Penicillium moulds from contaminated foodstuffs, old books, paintings, etc., but none except one provided by the hospital mycologist had the potency of the first mould. This confirmed his suspicion that he was dealing with a very unusual strain. 

---