Penicillin discovered by Sir Alexander Fleming

Sir Alexander Fleming was a young bacteriologist when an accidental discovery led to one of the great developments of modern medicine on September 3, 1928.

While working at St Mary's Hospital, London, Flemingmwas investigating the pattern of variation in S. aureus. He was inspired by the discovery of an Irish physician, Joseph Warwick Bigger, and his two students, C.R. Boland and R.A.Q. O’Meara, at Trinity College, Dublin, in 1927. Bigger and his students found that when they cultured a particular strain of S. aureus, which they had designated "Y" and isolated a year earlier from the pus of a patient's axillary abscess, the bacterium grew into a variety of strains. They published their discovery as "Variant colonies of Staphylococcus aureus" in The Journal of Pathology and Bacteriology, concluding: 

We were surprised and rather disturbed to find, on a number of plates, various types of colonies which differed completely from the typical aureus colony. Some of these were quite white; some, either white or of the usual color were rough on the surface and with crenated margins. 

Fleming and his research scholar Daniel Merlin Pryce pursued this experiment but Pryce was transferred to another laboratory in early 1928. After a few months of working alone, a new scholar, Stuart Craddock, joined Fleming. Their experiment was successful and Fleming was planning and agreed to write a report in A System of Bacteriology to be published by the Medical Research Council (MRC) by the end of 1928. In August, Fleming spent the summer break with his family at his country home The Dhoon at Barton Mills, Suffolk. Before leaving his laboratory, he inoculated several culture plates with S. aureus. He kept the plates aside on one corner of the table away from direct sunlight and to make space for Craddock to work in his absence. While on holiday, he was appointed Professor of Bacteriology at the St Mary's Hospital Medical School on September 1, 1928. He arrived at his laboratory on September 3, where Pryce was waiting to greet him. As he and Pryce examined the culture plates, they found one with an open lid and the culture contaminated with a blue-green mold. In the contaminated plate the bacteria around the mold did not grow, while those farther away grew normally, meaning that the mold killed the bacteria. Fleming commented as he watched the plate: "That's funny". Pryce remarked to Fleming: "That's how you discovered lysozyme." Fleming photographed the culture and took a sample of the mold for identification before preserving the culture with formaldehyde.

Fleming resumed his vacation and returned in September. According to his notes on October 30, he collected the original mold and grew it in culture plates. After four days he found that the plates developed large colonies of the mold. He repeated the experiment with the same bacteria-killing results. He later recounted his experience: 

When I woke up just after dawn on September 28, 1928, I certainly didn't plan to revolutionize all medicine by discovering the world's first antibiotic, or bacteria killer. But I suppose that was exactly what I did.

In 1929, Fleming introduced his mold by-product called penicillin to cure bacterial infections.

Learn more at en.wikipedia.org/wiki/Discovery_of_penicillin