Before giving it the formal name penicillin, Fleming called the antibacterial substance “mould juice.”
Fleming made the observation in 1928 while working at St Mary's Hospital in London. A mould had contaminated a culture of Staphylococcus bacteria, and the bacteria failed to grow in a clear area surrounding the mould. Fleming investigated the effect and found that the mould released an antibacterial substance into the culture medium.
For several months, he referred to the substance informally as “mould juice” or “the inhibitor.” On 7 March 1929, he used the name penicillin, derived from Penicillium, the genus of mould associated with the discovery. The original mould was once identified as Penicillium notatum; modern research identifies Fleming's strain as Penicillium rubens.
A common mistake is to treat Fleming's discovery as the whole story of the antibiotic. Fleming demonstrated the effect and named it, but researchers at Oxford—including Howard Florey and Ernst Chain—later developed methods for producing and testing penicillin as a practical medicine. The three shared the 1945 Nobel Prize in Physiology or Medicine.