Penicillin was discovered when Sir Alexander Fleming noticed that Penicillium notatum could neutralize a bacterial colony of Staphylococcus aureus, according to About.com. Fleming made the discovery by mistake when he left a plate culture of the bacteria open for mold contamination unintentionally, claims Wikipedia.
French scientist Ernest Duchesne studied the link between mold and bacteria in 1897, but his research was ignored, notes Wikipedia. He demonstrated his research by curing guinea pigs that were suffering from typhoid.
Alexander Fleming was a bacteriologist who worked at St. Mary's Hospital in London, notes About.com. His research revealed that the antibacterial properties of the bluish-green mold could be used to eliminate bacteria that caused certain ailments.
Fleming grew the mold and discovered that it released a substance that neutralized a variety of diseases, reports About.com. However, Fleming's research was not deemed noteworthy at the time. He named the mold penicillin, and he published his research in 1929, believing that it could be an effective medicine if produced on a mass scale.
It was not until 1939 that Dr. Howard Florey and three other colleagues at Oxford University effectively proved that penicillin could kill bacteria, explains About.com. Since the scientists could not conduct clinical trials because of the war effort, they concluded their research in the United States. Penicillin was produced on a commercial scale in the early 1940s, and it was used to treat soldiers who fought on D-Day.