Alexander Fleming: Bacteriologist Who Discovered Penicillin

Alexander Fleming
Alexander Fleming.

By Official photographer [Public domain], via Wikimedia Commons

In 1928, Alexander Fleming (August 6, 1881 - March 11, 1955) discovered the antibiotic penicillin at Saint Mary's Hospital in London. The discovery of penicillin revolutionized our ability to treat bacterial-based diseases, allowing physicians all over the world to combat previously deadly and debilitating illnesses with a wide variety of antibiotics.

Fast Facts: Alexander Fleming

  • Full Name: Alexander Fleming
  • Known For: The discovery of penicillin and the discovery of lysozyme
  • Born: August 6, 1881, Lochfield, Ayrshire, Scotland.
  • Parent's Names: Hugh and Grace Fleming
  • Died: March 11, 1955 in London, England
  • Education: MBBS degree, St. Mary's Hospital Medical School
  • Key Accomplishments: Nobel Prize for Physiology or Medicine (1945)
  • Spouses' Names: Sarah Marion McElroy (1915 - 1949), a nurse, and Dr. Amalia Koutsouri-Voureka (1953 - 1955), a medical practitioner
  • Children's Names: Robert (with Sarah) who was also a medical doctor

Early Years

Alexander Fleming was born in Lochfield, in Ayrshire, in Scotland on August 6, 1881. He was the third child in the family of his father's second marriage. His parents' names were Hugh and Grace Fleming. Both were farmers and had a total of four children together. Hugh Fleming also had four children from his first marriage, so Alexander had four half siblings.

Alexander Fleming attended both the Louden Moor and Darvel Schools. He also attended Kilmarnock Academy. After moving to London, he attended the Regent Street Polytechnic school followed by St. Mary's Hospital Medical School.

From St. Mary's he earned an MBBS (Medicinae Baccalaureus, Baccalaureus Chirurgiae) degree in 1906. This degree is similar to earning an M.D. degree in the United States.

After graduation, Fleming took a job as a researcher in bacteriology under the guidance of Almroth Wright, an immunology expert. During this time, he also completed a degree in bacteriology in 1908.

Career and Research

During his time studying bacteriology, Fleming noticed that while people had bacterial infections, their bodies' immune system would typically fight off the infections. He became very interested in such learnings.

With the advent of World War I, Fleming enlisted and served in the Royal Army Medical Corps rising to the rank of captain. Here, he began to exhibit the brilliance and ingenuity that he would become known for.

During his time in the Army Medical Corps, he noticed that the antiseptic agents that were being used to fight infections in deep wounds were actually harmful, sometimes leading to the death of soldiers. In essence, the agents were interfering with the body's natural ability to fight infection.

Fleming's mentor, Almroth Wright, had previously thought that sterile salt water would be better to treat these deep wounds. Wright and Fleming advocated that the antiseptics were preventing the healing process and that a sterile saline solution was the better alternative. By some estimates, it took quite some time for the practice to catch on, resulting in additional casualties.

The Discovery of Lysozyme

After the war, Fleming continued his research. One day while he had a cold, some of his nose mucus fell into a bacterial culture. Over time, he noticed that the mucus appeared to stop bacterial growth.

He continued his study and discovered that there was a substance in his mucus that stopped bacteria from growing. He called the substance lysozyme. Ultimately, he was able to isolate a larger quantity of the enzyme. He was excited about its bacteria-inhibiting properties, but eventually determined that it was not effective across a wide range of bacteria.

The Discovery of Penicillin

In 1928, Fleming was still experimenting at St. Mary's Hospital in London. Many have described Fleming as not being too 'fastidious' when it came to the more technical aspects of keeping a clean laboratory environment. One day, after coming back from a vacation, he noticed that some type of mold had developed in a contaminated culture. The contaminated culture contained staphylococcus bacteria. Fleming noticed that the mold appeared to be inhibiting the growth of the bacteria. Inadvertently, Fleming had stumbled upon the antibiotic penicillin, a discovery that would revolutionize medicine and change how bacterial infections are treated.

How Penicillin Works

Penicillin works by interfering with the cell walls in bacteria, ultimately causing them to burst or lyse. The cell walls of bacteria contain substances called peptidoglycans. Peptidoglycans fortify bacteria and help prevent external objects from entering. Penicillin interferes with peptidoglycans in the cell wall, allowing water to come through, which eventually causes the cell to lyse (burst). Peptidoglycans are only present in bacteria and not in humans. That means that penicillin interferes with bacterial cells but not with human cells.

In 1945, Fleming, along with Ernst Chain and Howard Florey, was awarded the Nobel Prize in Physiology or Medicine for their work with penicillin. Chain and Florey were instrumental in testing the efficacy of penicillin after Fleming's discovery.

Death and Legacy

Over the course of time, certain seminal discoveries profoundly change the course of a particular discipline. Fleming's discovery of penicillin was one such discovery. It is difficult to overstate the magnitude of his impact: untold millions of lives have been saved and improved by antibiotics.

Fleming amassed a number of prestigious awards during his lifetime. He was awarded the John Scott Legacy Medal in 1944, the aforementioned Nobel Prize in Physiology or Medicine in 1945, as well as the Albert Medal in 1946. He was knighted by King George VI in 1944. He was a member of the Pontifical Academy of Science and was awarded the Hunterian Professorship by the Royal College of Surgeons of England.

Fleming died at home in London at the age of 73 of a heart attack.


  • Tan, Siang Yong, and Yvonne Tatsumura. Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, July 2015,
  • “The Nobel Prize in Physiology or Medicine 1945.”,
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Bailey, Regina. "Alexander Fleming: Bacteriologist Who Discovered Penicillin." ThoughtCo, Aug. 17, 2021, Bailey, Regina. (2021, August 17). Alexander Fleming: Bacteriologist Who Discovered Penicillin. Retrieved from Bailey, Regina. "Alexander Fleming: Bacteriologist Who Discovered Penicillin." ThoughtCo. (accessed June 10, 2023).