Penicillin is one of the most important medical discoveries in modern history because it turned many once-deadly bacterial infections into treatable illnesses. In 1928, Alexander Fleming noticed something unusual on a contaminated petri dish in his London laboratory. A mold called Penicillium had grown on the plate, and the bacteria near it had disappeared.
That clear bacteria-free ring became evidence that the mold released a substance able to kill or stop bacteria.
Key Facts
- Alexander Fleming observed the penicillin effect in 1928 at St. Mary's Hospital in London.
- The clear zone around the mold is called a zone of inhibition because bacteria failed to grow there.
- Penicillin is an antibiotic, meaning it targets bacteria rather than viruses.
- Penicillin weakens bacterial cell walls, causing many bacteria to burst or stop growing.
- Howard Florey, Ernst Chain, and their team helped purify and mass-produce penicillin in the late 1930s and early 1940s.
- Fleming, Florey, and Chain shared the 1945 Nobel Prize in Physiology or Medicine for penicillin.
Vocabulary
- Penicillin
- Penicillin is an antibiotic originally discovered from Penicillium mold that can kill or stop the growth of many bacteria.
- Antibiotic
- An antibiotic is a medicine that treats bacterial infections by killing bacteria or preventing them from multiplying.
- Petri dish
- A petri dish is a shallow laboratory dish used to grow microorganisms such as bacteria or mold.
- Zone of inhibition
- A zone of inhibition is the clear area around an antimicrobial substance where bacteria cannot grow.
- Penicillium
- Penicillium is a type of mold that can produce penicillin under the right conditions.
Common Mistakes to Avoid
- Saying Fleming immediately created a ready-to-use medicine is wrong because his 1928 observation was only the first step, and later scientists had to purify, test, and produce penicillin safely.
- Thinking penicillin works against viruses is wrong because penicillin targets bacterial cell walls, and viruses do not have bacterial cell walls.
- Ignoring the clear ring around the mold is a mistake because that zone of inhibition was the key visual evidence that the mold was affecting bacterial growth.
- Giving Fleming all the credit for wartime penicillin is incomplete because Florey, Chain, and other researchers played major roles in turning the discovery into a mass-produced medical treatment.
Practice Questions
- 1 Fleming observed the penicillin effect in 1928, and the Nobel Prize was awarded in 1945. How many years passed between the observation and the Nobel Prize?
- 2 A petri dish has a mold colony in the center and a circular bacteria-free zone with a radius of 2 cm. Using area = 3.14r^2, what is the area of the clear zone?
- 3 Explain why the discovery of penicillin is considered both a scientific accident and a medical breakthrough.