Alexander Fleming discovered penicillin, the world’s first antibiotic, in 1928. That single finding launched the era of antibiotic medicine and has saved an estimated hundreds of millions of lives since. But Fleming’s career involved more than one lucky moment in a London lab. His work spanned two world wars, multiple discoveries, and a decades-long effort to convince the scientific community that mold could cure deadly infections.
The Discovery of Penicillin
On September 28, 1928, Fleming returned to his laboratory at St. Mary’s Hospital in London after a vacation and noticed something unusual. A petri dish containing staphylococcal bacteria had been contaminated by a mold. Rather than discarding it, Fleming observed that the bacteria surrounding the mold had been killed, leaving a clear ring where nothing grew.
He isolated the mold and identified it as belonging to the Penicillium genus, then named its active bacteria-killing agent “penicillin.” Testing showed it was effective against a wide range of dangerous pathogens responsible for diseases like scarlet fever, pneumonia, meningitis, and diphtheria. As Fleming himself later put it: “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.”
Fleming recognized penicillin’s potential, but he faced a major problem. The substance was unstable and extremely difficult to produce in usable quantities. He published his findings, yet for more than a decade, little notice was given to the discovery, and the active substance was never successfully isolated in a pure, concentrated form.
From Lab Curiosity to Life-Saving Drug
Penicillin might have remained a footnote in bacteriology if not for two other scientists. In 1939, Howard Florey and Ernst Boris Chain at the University of Oxford led a team that finally figured out how to manufacture penicillin from the liquid broth in which it grows. They purified it, tested it on mice and then on human patients, and proved it could cure previously fatal infections.
By the early 1940s, with World War II creating desperate demand for infection treatment, British and American laboratories scaled up production. The results were dramatic. Among soldiers with abdominal wounds in 1944, switching from older antibacterial drugs to penicillin cut the death rate from bacterial infection of the abdominal lining from 5 percent down to 0.3 percent. The overall fatality rate for abdominal injuries dropped from about 33 percent to 22 percent over the course of that year as penicillin became standard treatment.
His Earlier Discovery: Lysozyme
Penicillin wasn’t Fleming’s first brush with natural antibacterial substances. In the early 1920s, he discovered lysozyme, an enzyme found naturally in human tears, saliva, and nasal secretions. He noticed that these bodily fluids could kill certain bacteria on contact, revealing that the human body has its own built-in defense system against microbes. Lysozyme turned out to be one of the body’s first lines of immune defense, though it wasn’t powerful enough to treat serious infections. The discovery did, however, prime Fleming’s thinking. It taught him to pay attention when biological substances destroyed bacteria, which is exactly the instinct that led him to investigate the contaminated petri dish six years later.
Nobel Prize and Global Recognition
In 1945, Fleming, Florey, and Chain shared the Nobel Prize in Physiology or Medicine “for the discovery of penicillin and its curative effect in various infectious diseases.” Fleming had already been knighted by King George VI the year before, becoming Sir Alexander Fleming. He received honorary degrees and memberships in scientific societies around the world.
The recognition transformed him personally. Initially described as shy, uncommunicative, and a poor lecturer, Fleming blossomed under the global attention and became one of the world’s most recognizable scientists. For the last decade of his life, he served as a kind of ambassador for medicine and science, traveling widely and speaking about his work. In 1946, he was named head of the department at St. Mary’s where he had spent his entire career. It was renamed the Wright-Fleming Institute in his honor.
Why Fleming’s Discovery Still Matters
Before penicillin, a simple scratch could kill you if it became infected. Surgeries carried enormous risk. Pneumonia, strep throat, and wound infections were leading causes of death. Penicillin and the antibiotics that followed changed the math of human survival so fundamentally that it’s easy to forget how recently these drugs arrived. Fleming’s observation in 1928 didn’t just add a new drug to the pharmacy. It opened an entirely new category of medicine, one that made modern surgery, cancer treatment, and organ transplantation possible by giving doctors a way to fight the bacterial infections that would otherwise make those procedures too dangerous to attempt.
Fleming spent his career at St. Mary’s Hospital Medical School, researching and teaching until the end of his life. He remained active in bacteriology, mentoring younger scientists and contributing to the growing field of antibiotic therapy that his discovery had created.