Fleming discovers the action of penicillin

In September 1928 Alexander Fleming returned to work at St Mary's Hospital in London and noticed an unusual plate of staphylococcus culture. A colony of mould had contaminated the medium, and around it a clear zone free of bacterial growth had formed. Fleming was not the first person to see antagonism between microorganisms, but he investigated the phenomenon, named the active filtrate penicillin, and published his results in 1929.

The original article describes which bacteria the filtrate acted against, how its activity changed, and why it might be useful in isolating particular microorganisms. It does not yet present penicillin as a finished systemic medicine. The substance was unstable and hard to purify, and Fleming's laboratory did not solve the problem of producing an adequate quantity.

The clinical breakthrough came through the work of Howard Florey and Ernst Chain's team at Oxford. The researchers developed purification, demonstrated its action in models of infection, and began treating people. British and American laboratories and industry then increased the efficiency of fermentation. In 1945 Fleming, Florey and Chain jointly received the Nobel Prize.

The significance of Fleming's observation is best understood as the first stage of a long process. An accidental contamination became a discovery because it was noticed, described and shared with others. Only the teams that followed turned the clue into a therapy that transformed the treatment of infection. The original article protects that history from the shorthand in which a lone scientist "invents an antibiotic" ready for patients one morning.

What the 1929 paper actually demonstrated

The first page of Fleming’s paper records the observation: around a contaminating mould colony, staphylococcal colonies became transparent and underwent lysis; Figure 1 also shows the result. On page 227 he introduced “penicillin” as the filtrate of a broth culture of that mould, not as a purified drug. Measurements on page 228 showed that a good sample completely inhibited staphylococci at dilutions of 1:500 or 1:800 after six to seven days of culture. Figures 3 and 4 then document the filtrate as a selective tool: it suppressed susceptible bacteria and made Pfeiffer’s bacillus easier to isolate. In the summary on page 236 Fleming suggested a possible antiseptic application, while the firmly demonstrated use was chiefly bacteriological.