Final answer:
Alexander Fleming is credited with penicillin's discovery in 1928, and his work, along with that of Howard Florey and Ernst Chain who later purified and mass-produced it, led to them receiving the Nobel Prize in Physiology and Medicine in 1945.
Step-by-step explanation:
Penicillin was discovered by Alexander Fleming in 1928. This groundbreaking discovery occurred when Fleming observed antibacterial properties in a mold growing on a bacterial culture plate in his lab, leading to the identification of the world's first natural antibiotic. Although Fleming is credited with the discovery, it was Howard Florey and Ernst Chain who later performed significant work to purify penicillin and scale up its production, enabling it to become an effective antimicrobial agent in both animal and human trials. This collective effort was so monumental that Fleming, Florey, and Chain were awarded the Nobel Prize in Physiology and Medicine in 1945 for their contributions.