The first known antibiotic, penicillin, was discovered when Alexander Fleming noticed that a dangerous bacteria was not growing around a patch of mold in a petri dish. However, the initial strain of penicillin he developed was unstable and he did not focus on its use in medicine.
Fleming was growing Staphylococcus, which causes abscesses and boils in humans. The mold grew accidentally while he was away. His initial paper reporting his find focused on its use for separating various types of bacteria. Other researchers, including Howard Florey and Ernst Chain, saw its potential therapeutic benefits and began working to further isolate and stabilize the compound.
The use of mold to inhibit bacterial growth was known prior to Fleming's discovery, but the exact mechanism was unknown and mold itself contains dangerous compounds. Ancient Egyptians treated wounds by binding a piece of moldy bread to it, perhaps for this reason. Some ancient mummies have been found to have high levels of naturally occurring antibiotics, such as tetracycline, as well. Joseph Lister had noticed that urine samples that contained mold often did not contain much bacteria in 1871, but he was unable to determine why this occurred.
After penicillin was developed, researchers began looking for other sources of antibiotics. Scientists have isolated antibiotic compounds produced by certain types of bacteria and other microbes, as well as by other plants and natural sources.