Science & Discovery

Explore the fascinating world of science through groundbreaking discoveries, unexpected findings, and innovations that have transformed our lives. Discover the stories behind scientific breakthroughs and the curiosity, creativity, and perseverance that drive scientific progress. Our goal is to inspire young minds to ask questions, think critically, and explore the world around them. 

🧬 Today in History | September 28, 1928

How a Contaminated Petri Dish Changed the History of Medicine

Published: September 28, 2026 | BioMedEdu

On September 28, 1928, the traditionally recognized date of his discovery, Scottish bacteriologist Alexander Fleming observed the antibacterial effects of a mold at St. Mary’s Hospital in London. This discovery led to the development of penicillin, marking the beginning of a new era in medicine and transforming the treatment of bacterial infections. 

🔬 1. An Accidental Discovery

Fleming was studying Staphylococcus bacteria. When he returned from vacation, he noticed that one of his Petri dishes had been contaminated with mold.

Something unusual caught his attention: the bacteria surrounding the mold had disappeared, leaving a clear area known as a zone of inhibition.

Fleming realized that the mold might be producing a substance capable of preventing bacterial growth.

He identified the mold as belonging to the genus Penicillium and named the antibacterial substance it produced penicillin.

🧪 2. From Laboratory Discovery to Life-Saving Medicine

Discovering penicillin was only the beginning. Turning it into a medicine that could safely treat patients was a much greater challenge.

Penicillin was difficult to purify and chemically unstable. Although Fleming recognized its antibacterial properties, he could not develop it into a practical treatment.

In the late 1930s and early 1940s, Howard Florey, Ernst Chain, and their colleagues at the University of Oxford made crucial breakthroughs. They developed methods to extract and purify penicillin and demonstrated its effectiveness through animal experiments and clinical studies.

During World War II, large-scale production of penicillin, particularly in the United States, made it widely available for treating bacterial infections and saved countless lives.

In 1945, Fleming, Florey, and Chain shared the Nobel Prize in Physiology or Medicine.

🦠 3. How Does Penicillin Kill Bacteria?

Penicillin belongs to a group of antibiotics called beta-lactams.

Many bacteria have a protective cell wall made of peptidoglycan. This wall helps maintain the bacterial cell's shape and prevents it from bursting under internal osmotic pressure.

Penicillin works by binding to proteins called penicillin-binding proteins (PBPs), which are essential for building and maintaining the bacterial cell wall.

By interfering with cell wall synthesis, penicillin weakens the walls of susceptible bacteria. As the bacteria grow and divide, they may eventually rupture and die.

Why doesn't penicillin kill human cells?

Human cells do not have peptidoglycan cell walls. This difference allows penicillin to selectively target bacteria without disrupting the same structure in human cells.

However, penicillin does not work against all bacteria, and it cannot treat viral infections such as influenza or the common cold.

💡 4. What Can We Learn From Fleming's Discovery?

Fleming's discovery is often described as a lucky accident. But the real lesson goes beyond luck.

Many scientists might have discarded a contaminated Petri dish. Fleming noticed something unexpected, asked why it happened, and investigated further.

His discovery also illustrates another important principle: a scientific breakthrough does not necessarily become a useful technology immediately.

It took more than a decade of additional research, collaboration, clinical testing, and industrial development to turn penicillin into a widely available medicine.

The discovery of penicillin demonstrates how observation, curiosity, experimental investigation, and teamwork can transform an unexpected finding into a major medical advancement.

🎓 Three Questions for High School Students

1. Scientific observation: If you were Fleming and discovered mold growing in your bacterial culture, would you discard the Petri dish or investigate further? Why?

2. Cell biology: Why can penicillin kill many bacteria but not viruses? What does this tell us about the differences between bacterial and viral structures?

3. Evolution and antibiotic resistance: If penicillin can kill bacteria, why have some bacteria evolved resistance to it? How does natural selection contribute to antibiotic resistance?

The discovery of penicillin reminds us that scientific breakthroughs sometimes begin with unexpected observations. Turning those observations into discoveries requires curiosity, careful experimentation, and the willingness to ask questions.

Science is not just about discovering the unknown. It is about transforming discoveries into advances that improve and save human lives.

References

1. Fleming A. On the antibacterial action of cultures of a Penicillium, with special reference to their use in the isolation of B. influenzae. British Journal of Experimental Pathology. 1929;10(3):226–236. Original research paper.

2. Nobel Prize Outreach. The Nobel Prize in Physiology or Medicine 1945. NobelPrize.org.

3. Gaynes R. The discovery of penicillin—new insights after more than 75 years of clinical use. Emerging Infectious Diseases. 2017;23(5):849–853. Read article.

4. Kong KF, Schneper L, Mathee K. Beta-lactam antibiotics: from antibiosis to resistance and bacteriology. APMIS. 2010;118(1):1–36. Read article.