How did Alexander Fleming contribute to Science?

Discovered penicillin  

Explanation:

Alexander Fleming accidentally discovered penicillin in 1928. He left dirty petri dishes in his laboratory. When he did clean them, he discovered a mold growth, and around this mold, no bacteria were present. There were bacteria in other regions, but none around the growth. He then knew that he discovered an antibiotic: penicillin.

He received the Nobel Prize in Physiology or Medicine in 1945 for his findings. Today, penicillin is used to fight bacterial infections such as pneumonia.

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