The mold that contaminated Alexander Fleming’s bacterial culture plate in 1928 was Penicillium notatum. Fleming noticed that bacteria failed to grow near the mold, revealing an antibacterial substance he called penicillin.
Fleming was studying staphylococci at St Mary’s Hospital in London when he returned from holiday and examined a culture left on a laboratory bench. The contamination was accidental, but the important observation was deliberate: he investigated the clear zone around the mold instead of discarding the plate.
He published his findings in 1929 and showed that the mold’s product could inhibit several bacteria. Fleming did not turn penicillin into a practical medicine by himself. Howard Florey, Ernst Chain, and their colleagues later developed methods for purification, testing, and large-scale production during the 1940s.
The species name is commonly given as P. notatum in historical accounts, although later taxonomic work has associated Fleming’s strain with Penicillium rubens. The discovery and the drug-development process were separate achievements.