Penicillin was discovered by Alexander Fleming from the Penicillium mould.

The idea

In 1928 Alexander Fleming noticed that a mould contaminating a culture plate of bacteria had killed the bacteria around it. He identified the mould as Penicillium and named the antibacterial substance it produced penicillin. This was the first antibiotic discovered. Fleming did not purify penicillin or turn it into a usable medicine; that required later work by Florey and Chain, who mass-produced it during the 1940s. The discovery shows how a chance observation, followed by careful scientific investigation, can lead to a life-saving drug. Penicillin works by interfering with bacterial cell wall formation, so it kills bacteria without directly damaging human cells. It is effective only against bacteria, not viruses, and some bacteria have evolved resistance, so antibiotics must be used carefully.

What earns the marks

• State that Alexander Fleming discovered penicillin in 1928 from the Penicillium mould.
• Describe that the mould produced a substance that killed bacteria growing on the culture plate.
• Name penicillin as an antibiotic that kills bacteria and is used to treat bacterial infections.
• Recognise that Fleming's discovery was a chance observation followed by scientific investigation.
• Explain that penicillin was later developed into a usable medicine by other scientists, such as Florey and Chain.
• Distinguish between discovery (Fleming) and development or mass production (later scientists).

Common mistakes and how to put them right

• Saying Fleming invented or mass-produced penicillin.

  Put it right: Fleming discovered penicillin; Florey and Chain later developed it into a medicine.

• Claiming penicillin kills viruses.

  Put it right: Penicillin is an antibiotic and acts only on bacteria, not viruses.

• Stating that Fleming deliberately set out to find an antibiotic.

  Put it right: The discovery followed an accidental contamination of a culture plate.

• Writing that penicillin was discovered from a bacterium.

  Put it right: It was produced by the Penicillium mould.

Exam tips

• Link the scientist to the specific action: Fleming observed, identified the mould and named penicillin.
• Use the word antibiotic correctly and state that it treats bacterial infections.
• If asked about drug development, separate discovery from later purification, testing and mass production.
• Check that you have not written that Fleming made penicillin a usable medicine; credit usually depends on the correct role.