Alexander Fleming, Penicillium chrysogenum (P.notatum) and Staphylococcus aureus
Staphylococcus aureus played an important role in discovery of penicillin G (benzylpenicillin). As Alexander Fleming writes in his Nobel lecture, December 11, 1945:
"The origin of penicillin was the contamination of a culture plate of staphylococci by a mould. It was noticed that for some distance around the mould colony the staphylococcal colonies had become translucent and evidently lysis was going on. This was an extraordinary appearance and seemed to demand investigation, so the mould was isolated in pure culture and some of its properties were determined. The mould was found to belong to the genus Penicillium and it was eventually identified as Penicillium notatum,..."
"It arose simply from a fortunate occurrence which happened when I was working on a purely academic bacteriological problem which had nothing to do with antagonism, or moulds, or antiseptics, or antibiotics."
"...penicillin started as a chance observation. My only merit is that I did not neglect the observation and that I pursued the subject as a bacteriologist. My publication in 1929 was the starting-point of the work of others who developed penicillin especially in the chemical field."
This "fortunate occurrence" (and more or less fortunate occurrences that followed) led a team of Howard W. Florey (Ernst Chain, Norman Heatley and other scientists) to the mass production of penicillin.
It is impossible to know how many lives have been saved by penicillin but it is estimated that penicillin saved 80.000.000 to 200.000.000 lives. Penicillin has saved, and is still saving, millions of people around the world.
To understand what these numbers mean it's good to realize that the total casualties of the World War II were about 75 million people.