Final answer:
Penicillin was discovered by accident by Alexander Fleming when he left a petri dish of Staphylococcus bacteria open. The mold that grew on the dish turned out to be Penicillium, leading to the discovery of a new antibiotic.
Step-by-step explanation:
Penicillin was discovered by accident by Alexander Fleming when he left a petri dish of Staphylococcus bacteria open.
An unwanted mold grew on the dish, killing the bacteria.
The mold turned out to be Penicillium, and a new critically important antibiotic was discovered.
It was later isolated and mass-produced by a team of researchers at Oxford University.